Updated at 4:43 p.m. ET: BEIRUT, Lebanon -- A huge car bomb explosion in Beirut on Friday killed a top Lebanese security official whose investigations implicated Syria and Hezbollah in the assassination of former Lebanese prime minister Rafik al-Hariri seven years ago.

The rush-hour bomb in the center of the Lebanese capital killed eight people and wounded about 80 others, heightening fears that Syria's war is spilling over into Lebanon.

Among the dead was Wissam al-Hassan, the head of a Lebanese intelligence agency who had also uncovered a recent bomb plot that led to the arrest of a pro-Syrian Lebanese politician, a Lebanese official said.

Al-Hassan was a close aide to Hariri, a Sunni Muslim who was killed in a 2005 bomb attack in downtown Beirut. Al-Hassan's investigation into Hariri's death uncovered evidence that implicated Syria and Hezbollah in the killing.

Follow this story at BreakingNews.com

It was also not clear if the explosion targeted any political figure in Lebanon's divided community but it occurred at a time of heightened tension between Lebanese factions on opposite sides of the Syria conflict.

Ambulances rushed to the scene in the Ashafriyeh district, a mostly Christian area, as smoke rose from the area. 

The explosion ripped through the street where the office of the anti-Damascus Christian Phalange Party is located near Sassine Square.

Reuters

Phalange leader Sami al-Gemayel, a staunch opponent of Syrian President Bashar al-Assad and a member of parliament, condemned the attack.

"Let the state protect the citizens. We will not accept any procrastination in this matter, we cannot continue like that. We have been warning for a year. Enough," said Gemayel, whose brother was assassinated in November 2006.

Several cars were set on fire by the explosion and the front of a multi-story building was badly damaged. Residents ran about in panic looking for relatives while others helped carry the wounded to ambulances, Reuters reported. 

Slideshow: Bombing in Beirut

Reuters

Huge blast explodes in a central Beirut street injures dozens, kills at least eight.

Launch slideshow

Pope tells Christians in Beirut: 'Be peacemakers'

Security forces blanketed the area.

Witness Danny Rizkallah told NBC News the blast took place close to the headquarters of a Lebanese opposition political party with links to Syria rebels and close to the scene of the 1982 assassination of then president-elect Bachir Gemayel. The affluent, largely Christian, district is also home to the American University of Science and Technology (AUST).

He said he was having lunch nearby when the blast lifted him from his chair. “It was an incredibly powerful explosion,” he said. “I knew immediately it was a bomb because it has such a different sound to shelling.”

“I rushed around the corner to see what happened there were lots of people injured by broken glass from the windows of nearby stores. It did a great deal of damage to nearby buildings and there was a lot of glass.

Hasan Shaaban / Reuters

Burning cars and damages are seen at the site of an explosion in Ashafriyeh, central Beirut, October 19, 2012.

“For this to happen is shocking because we really thought this sort of thing had stopped in Beirut, and for it to happen in the Christian district is also very unusual. I really don’t know who is behind this, or why. Our politics is very messed-up.”

The last bombing in Beirut was in 2008 when three people were killed in an explosion that damaged a U.S. diplomatic car. 

U.S. officials are condemning the attack "in the strongest terms," calling the blast a terrorist attack.

"We condemn this act of terrorism," State Department spokeswoman Victoria Nuland said.

"There is no justification for such violence," she added. "We obviously express our heartfelt sympathies for the families and the loved ones of those who were killed and injured, and we stand by the people of Lebanon and renew our commitment to a stable, sovereign, and independent Lebanon."

National Security Council spokesman Tommy Vietor said in a statement there is "no justification for using assassination as a political tool." He says the U.S. will stand with the Lebanese government to bring to justice those responsible "for this barbaric attack."

Sunni-Shiite tensions
Tension between Sunnis and Shiites has been rumbling in Lebanon ever since the end of the 1975-1990 civil war but reignited after the Syria conflict erupted.

It reached its peak when Hariri, a Sunni, was killed in 2005. Hariri supporters accused Syria and then Hezbollah of killing him -- a charge they both deny. An international tribunal accused several Hezbollah members of involvement in the murder.

Clashes over Syrian conflict in Lebanon leave ten dead

Hezbollah's political opponents, who have for months accused it of aiding Assad's forces -- have warned that its involvement in Syria could ignite sectarian tension of the civil war. 

However fighting had broken out this year between supporters and opponents of Assad in the northern city of Tripoli.

Reuters, The Associated Press and NBC News' Paul Nassar contributed to this report.

More world stories from NBC News:

• Self-professed Sept. 11 mastermind Mohammed airs his views at Gitmo hearing
• British government to recruit teens as next generation of spies
• U.S. nonprofit 'names and shames' businesses to put bite into Iran sanctions
• Van full of bodies stolen during drivers' break in Germany
• Revolt of the underclass in Syria
• Fidel Castro statement read at Havana event amid rumors about his health
• Rights group blasts Rwanda winning seat on UN Security Council
• 'Spy of the West': Al-Qaida, Taliban struggle to justify attack on Pakistani teen
• UK computer hacker wins 10 year fight against extradition to US

Follow World News from NBCNews.com on Twitter and Facebook

Updated at 12:00 p.m. ET: Russia rebuffed President Barack Obama's threat of unilateral action against Syria Tuesday, as officials said 2,500 refugees fled across the border into Turkey in just 24 hours – one of the highest daily refugee flows of recent weeks.

Russian Foreign Minister Sergei Lavrov, speaking after meeting China's top diplomat, said Moscow and Beijing were committed to "the need to strictly adhere to the norms of international law ... and not to allow their violation".

Obama draws 'red line' for Syria on chemical and biological weapons

Obama on Monday threatened "enormous consequences" if his Syrian counterpart used chemical or biological arms or even moved them in a menacing way.

The president used some of his strongest language yet to warn Assad not to use chemical or biological weapons – after Syria acknowledged for the first time that it had such weapons and could use them if foreign countries attacked it.

"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is (if) we start seeing a whole bunch of chemical weapons moving around or being utilized," he said. "That would change my calculus."

Syria 'ready to discuss' Assad's resignation, deputy PM says

"We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people," Obama said, perhaps referring to Lebanon's Shiite Hezbollah group, an Iranian-backed ally of Assad, or to Islamist militants.

Turkey's foreign minister has warned it can accommodate no more than 100,000 refugees and that the United Nations may need to create a "safe zone" within Syria to shelter any beyond that number.

Slideshow: Syria uprising

Goran Tomasevic / Reuters

After months of protests and violent crackdowns, a look back at the violence that has overtaken the country.

Launch slideshow

Thousands of refugees
A Turkish official told Reuters on Tuesday that about 2,500 people fleeing violence in Syria had entered Turkey in the preceding 24 hours, most of them entering the southeastern Turkish province of Hatay.

Turkish journalist Mahir Zeynalov reported on Twitter that four Syrian colonels and two captains crossed the border early Wednesday.

PhotoBlog: Clashes over Syrian conflict in Lebanon leave ten dead

Turkey is now sheltering close to 70,000 Syrian refugees and is struggling to accommodate the influx, which rose after a bomb attack near the border killed eight, spreading panic.

In Lebanon, street battles between Sunnis and Alawites continued for a second night running, fueled by conflicting loyalties in the conflict across the border. The BBC reported that seven were killed and more than 70 wounded in the country's second-largest city, Tripoli.

Syrian President Bashar Assad, an Alawite, is battling largely Sunni opposition fighters. Lebanese Prime Minister Najib Mikati, himself a Sunni, appealed to both sides to end the "absurd battle" in Tripoli.

In Syria itself, the army deployed tanks on a ring road surrounding Damascus on Wednesday and shelled southern neighborhoods where rebels operate, in the heaviest bombardment on the capital since the army reasserted control last month, residents said.

At least eight people were killed in the shelling, which was accompanied by an aerial bombardment, on the Kfar Souseh, Daraya, Qadam and Nahr Aisheh neighborhoods, they told Reuters.

Regional news channel Al-Jazeera reported that at least 24 people were killed across the country on Tuesday, among them women and children in Aleppo - the city over which the rebel Free Syrian Army (FSA) claims two-thirds control, and where a Japanese journalist was killed on Monday.

Activists: Japanese journalist killed in Aleppo

"We now control more than 60 per cent of the city of Aleppo, and each day we take control of new districts," said Abdel Jabbar al-Okaidi, a colonel with the FSA. He went on to list some 30 districts which he claimed were under FSA control, including about half of the embattled neighborhood of Salaheddin.

But a security source in Damascus rejected the claims, according to the AFP news agency, calling them "completely false".

Likened to Iraq invasion
Syrian soldiers killed a journalist sympathetic to the rebels during a raid in Damascus on Wednesday. Mosaab al-Odaallah, who worked for the state-run Tishreen newspaper, was shot at point-blank range at his home by troops conducting house-to-house raids in the southern Nahr Eisha district of the capital, opposition activists said.

Massoud Akko, head of the public freedoms committee at the underground Syrian Journalists Association, said Odaallah's death brought to 54 the number of Syrian journalists, bloggers and writers killed by security forces during the uprising.

"Most have been killed with shots to the head. The regime appears to have adopted a systematic policy of killing journalists and social media activists," Akko told Reuters by telephone from Berlin.

Earlier, Syria's deputy Prime Minister Qadri Jamil said Obama's talk of action against Syria was media fodder.

Speaking after the news conference held by Russia's Lavrov, Jamil said the West was seeking an excuse to intervene, likening the focus on Syria's chemical weapons with the run-up to the 2003 invasion of Iraq by U.S.-led forces and the focus on what proved to be groundless suspicions that Saddam Hussein was concealing weapons of mass destruction.

"Direct military intervention in Syria is impossible because whoever thinks about it ... is heading towards a confrontation wider than Syria's borders," he told a news conference in Damascus.

Meanwhile, the Los Angeles Times reported the concerns of Christians, who make up about 10 per cent of Syria's population. It said Christians fleeing the fighting have detected an increasingly radicalized Islamist strain among the rebels that makes them fear for their future.

Reuters contributed to this report.

More world stories from NBC News:

• Israelis fret over 'lynching' of Palestinian
• Video: Poaching surge threatens survival of rhinos
• Anti-tanning 'Facekinis' cause stir on China beach
• Reports: Kim Jong Un will travel to Iran
• Slideshow: Migration in the Americas
• Reports: Olympic sprinter drowned when migrant boat sank
• With wife's conviction, what is next for China's Bo Xilai?

Follow World News from NBCNews.com on Twitter and Facebook

TRIPOLI, Lebanon – It only takes a two-minute stroll down Syria Street to see why so many people are so worried about what might happen next in Lebanon.

A hole punched through the wall of the mosque by a rocket or mortar shell, smoke-blackened masonry, shops and apartments bearing the pockmarks of fierce gun battles.

Syria Street is the aptly named thoroughfare that separates rival factions in Lebanon’s second city.

For much of the past week, the two sides have been waging a mini-civil war.

It is a direct spill over from the chaos in neighboring Syria.

Photos: Violence on the streets of Tripoli

One side of the street is home to a hard-line Sunni Muslim militia who run guns to rebels across the border.

“President Assad is trying to destroy us,” says Sheik Bilal Masri, by way of explanation. “They cause trouble here to take the pressure of them in Damascus.”

We meet a small group of his men. They are well-armed and apparently spoiling for a fight.

Not many yards away, posters of Syria’s President Bashar Assad striking stern military poses adorn walls on the other side of the street.

Here the people share Assad’s Alawite faith and, it seems, the same determination to defend his regime.

Omar Ibrahim / Reuters

A man hides behind sandbags amid clashes in the Bab al-Tebbaneh neighborhood in Tripoli, Lebanon, on Thursday.

“No one wants a civil war in Lebanon,” a local Alawite leader tells me.  “But everyone should be warned: There will be repercussion for anyone who tries to meddle in Syria.”

Conflict along Syria Street is nothing new. But the outside world began to take notice on Monday when for the first time in four years, gun battles broke out on the streets of Lebanon’s capital, Beirut.

2 killed, 18 hurt as Syria conflict spills over into Lebanon

It was a brief glimpse back into the abyss for a nation scarred by years of civil strife.

In 2005, Syrian troops were forced to withdrawal from Lebanon, but Damascus is still a big player in the fractured politics of a country that sees rival Muslim and Christian sects share power in a set of uneasy alliances.

Syria’s most powerful friend here is Hezbollah, the militant Shiite group that probably holds the key to whether Lebanon survives in one piece.

Inside Syria rebel stronghold: 'The city is on mute' 

Its heartland in the south of Beirut has been tense, but so far its leader, Hassan Nasrallah, has kept his forces out of the fray.

But for how long?

The fatal shooting of two Sunni clerics followed by the kidnapping of Lebanese Shiite pilgrims in Syria shows how unpredictable events have become.

A message to Assad? War games held near border

For more than two decades, Timur Goksel has watched events in Lebanon. Once of the U.N. Mission here, he now lectures at the American University in Beirut.

He tells me the country has rarely felt so dangerous.

“I hope I am wrong because this is scary. If the faction leaders lose control of these young guys with the guns then we’re in trouble,” he said.

Their bloody history has taught the Lebanese to be a fatalistic people.

“The country is at boiling point,” another seasoned observer told me with a shrug.  “What is coming will be very bad.”

More world news from msnbc.com and NBC News:

• Africa's Rainbow Nation troubled by racist time warp
• 'Nearly empty': A rare glimpse inside Syria rebel stronghold
• Terror suspect's eye color? UK's flying cameras know
• Analysis: How Egypt's election can transform the Middle East
• Tokyo Sky Tree takes root as world's second-tallest structure
• Robotic 'fish' takes to seas to catch pollution sooner

Follow us on Twitter: @msnbc_world

Syrian refugees wait for their turn to receive humanitarian aid at the entrance of an NGO in the area of Wadi Khaled on the Lebanese-Syrian border in northern Lebanon on Feb. 26, 2012.

TRIPOLI, Lebanon – They are just 55 miles away, but for Syrian refugees now in Tripoli, Lebanon, couldn't be more different.

We spent a cold and rainy day in Lebanon's north, crisscrossing from hospitals, to apartments to slums, meeting with Syrians fleeing their country and seeking refuge in Lebanon.

A 27-year-old patient, who spoke on the condition of anonymity, said he was shot in the leg by a sniper’s bullet. The wound was so severe, he couldn't get the proper treatment inside Baba Amr. Afraid to go to any hospitals inside Syria for fear of being turned over to government forces, he and his brother decided to make the trek to Lebanon. For four days they moved by car from house to house under cover of darkness and the constant barrage of war all around them.

When they crossed the border they were taken by activists to a hospital in Tripoli, Lebanon’s second largest city, but it was too late. The leg was severely infected and doctors had to amputate it at the knee.

For a country that over the years has seen its own share of violence, forcing many of its own citizens to take refuge in Syria, it's new for Lebanese to see Syrian refugees in their country. So much so that international aid workers and activists say Lebanon has been slow to acknowledge and deal with the flow of Syrians across the border into their country.

Part of problem, Syrian activists say, is the attempt by the Lebanese government to remain on the sidelines of the conflict – without conceding that its side effects are beginning to seep in.   

More than 7,000 Syrians refugees have fled into Lebanon and registered with the United Nations High Commissioner for Refugees.  The majority of them have crossed into the north of Lebanon, activists tell us.

Dangerous trek
Over the past few days, dozens of injured residents of the besieged cities of Baba Amr and Homs have made the dangerous trek across the border. None of those we interviewed agreed to show their faces on camera. All were reluctant to give us their real names fearing their family members still living inside Syria would be hunted down.

Another refugee who called himself Abu Fares saw the war in Syria spreading five months ago and decided to flee the country with his family. Back then, Syrian officials didn't object to single families exiting all together. Now, activists say, Syrian border guards will turn back families that appear to be "fleeing" the country. More families have taken the route of entering the country illegally, making it difficult to keep an accurate number of who has entered Lebanon.

Stringer / Reuters

Syrian refugees take part in a protest to call for international protection for Syria's anti-government protesters and better living conditions for Syrian refugees in Lebanon, in front of the Red Cross offices in Tripoli, northern Lebanon on Feb. 26, 2012.

So far, no large refugee camps have been set up inside Lebanon for displaced Syrians –  unlike in neighboring Jordan, which has also taken in thousands of refugees.

Instead, what has emerged is an acute housing crisis for the families currently in Lebanon. The majority of families have taken refuge in apartments in rundown buildings, often at exuberant prices.

Abu Fares and his family of nine are living in a small shack in an illegal seaside slum in Tripoli. Without any heat or regular electricity, they have struggled to survive, relying instead on handouts for clothes, blankets and medicine. His heart and back conditions have made it impossible for him to work in the low-paying, labor intensive jobs most Syrians can vie for.
                                                                                                                            
Puddles of water filled the narrow walkways in between the shacks, and makeshift wiring and electric cables crisscrossed the alleys to the small, cramped and humid huts. Despite the hardship, Abu Fares said he has no regrets that he fled Syria and said he has no plans to return until the Assad regime steps down from power.

Not really a welcome mat
For Syrian activists, Lebanon has proven to be a dangerous country to operate. Lebanon’s weak central government has failed to fully embrace other Arab countries and international calls for Assad to step down. Lebanon for now has opted to remain impartial in the conflict, tacitly allowing refugees to come into the country, but not allowing the opposition to openly equip the Free Syrian Army.

Instead, Syrian opposition activists say they are routinely harassed by Lebanese security forces and military intelligence.

More importantly, Syria's strongest Lebanese ally, Hezbollah, has acted as a counter-balance to any major and visible opposition taking root publicly in Lebanon. Hezbollah commands a strong street presence in Lebanon and can easily mobilize large crowds in support of the embattled Syrian president. 

Instead, Syrian opposition activists feel more comfortable that their leadership remains in Turkey and abroad. They say Lebanon's past relations with Syria make it easy for Syrian intelligence and pro-Assad operatives to target them. Still, activists are discreetly using Lebanon as a base from which to supply and arm their comrades inside the country.

Even if the government in Lebanon has been reluctant to take sides in the conflict, it may not be long before the conflict forces Lebanon into a more direct course of action.