Senior Lebanese intelligence official Wissam al-Hassan, who led the investigation that implicated Syria and Hezbollah in the assassination of former prime minister Rafik al-Hariri, was killed by a huge car bomb in Beirut on Friday.
The bomb, which exploded in a busy street during rush hour, killed seven other people and wounded about 80, officials said. The attack prompted Sunni Muslims to take to the streets in areas across the country, burning tires in protest.
Rubble and the twisted, burning wreckage of several cars filled the central Beirut street where the bomb exploded, ripping the facades and balconies off buildings. Firefighters scrambled through the debris and rescue workers carried off the bloodied victims on stretchers. The blast occurred when many parents were picking up children from school.
The attack brought the war in neighboring Syria to the Lebanese capital, confirming fears that the conflict would infect it neighbors.
The war in Syria, which has killed 30,000 people in the past 19 months, has pitted mostly Sunni insurgents against President Bashar al-Assad, who is from the Alawite sect linked to Shi'ite Islam.
Lebanon's religious communities are divided between those supporting Assad and those backing the rebels trying to overthrow him.
Hassan was a leading opponent of Assad within the Lebanese intelligence services.
"I can just say that it is true, he is dead," an official who worked with Hassan told Reuters.
Hassan was also the brains behind uncovering a bomb plot that led to the arrest in August of a Lebanese politician allied to Assad, a major setback for Damascus.
He had been a close aide to Hariri, a Sunni Muslim who was killed in a 2005 bomb attack. He led the investigation into the murder and uncovered evidence that implicated Syria and Hezbollah, Lebanon's pro-Iranian Shi'ite Muslim group.
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.
"His killing means striking the head. The (anti-Assad) officials are all exposed now and in danger of assassination. It will be easy to assassinate them now or they will have to leave the country. He was their protector," the official said.
BLOOD ON THE STREETS
The bombing, which was reminiscent of grim scenes from Lebanon's own 1975-1990 civil war, ripped through the street where the office of the anti-Damascus Christian Phalange Party is located near Sassine Square in Ashrafiyeh, a mostly Christian area.
Phalange leader Sami al-Gemayel, a staunch opponent of 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 2006.
In the aftermath of Friday's bomb, residents ran about in panic looking for relatives as security forces blanketed the area. Ambulances ferried the wounded to hospitals, which put out an appeal for blood donations.
An employee of a bank on the street pointed to the blown-out windows of his building.
"Some people were wounded from my bank. I think it was a car bomb. The whole car jumped five floors into the air," he said.
Lebanese Prime Minister Najib Mikati said the government was trying to find out who carried out the attack and said those responsible would be punished.
The prospect that Syria's war might spread to Lebanon has worried many people here and fighting broke out in February between supporters and opponents of Assad in the northern city of Tripoli.
Syria had long played a major role in Lebanese politics, siding with different factions during the civil war. It deployed troops in Beirut and parts of the country during the war and they stayed until 2005.
In Damascus, Syrian Information Minister Omran al-Zoabie told reporters: "We condemn this terrorist explosion and all these explosions wherever they happen. Nothing justifies them."
Tension between Sunnis, Shi'ites and Christians in Lebanon has continued after the civil war but has increased with the Syria conflict.
Khattar Abou Diab, a Middle East expert at the University of Paris, said the attack was clearly linked to the Syria crisis and Hassan was one of the few security chiefs protecting Lebanon's sovereignty and independence.
"He wasn't just an ordinary person. Since 2005, he was a close ally of Rafik al-Hariri and it is a major loss for Lebanon.
"This is now revenge against a man who confronted the Syrians and revenge against a district, a Christian district in the heart of Beirut. Regional powers are fighting in Syria and now also want to fight in Lebanon," he said.
Hezbollah's political opponents, who have for months accused it of aiding Assad's forces, have warned that its involvement in Syria could reignite the sectarian tension of the civil war.
"They warned of the implications of the Syrian crisis and here it comes," said Nabil Boumonsef, a columnist at the Lebanese newspaper An-Nahar.
"The explosion shows that Lebanon cannot be safe and peaceful in the middle of this situation boiling around it ... They are dragging in Lebanon so that it becomes a conflict arena," he told Reuters.
Bombings were a hallmark of the civil war but the last such attack in Beirut was in 2008 when three people were killed in an explosion which damaged a U.S. diplomatic car.
Beirut has undergone massive reconstruction to repair the damage from the civil war and in recent years has enjoyed a tourist boom, boosted by Beirut's pulsating nightlife. That source of revenue, crucial to Lebanon's prosperity, is now also under threat.
Reporting by Mariam Karouny, Oliver Holmes, Laila Bassam and Samia Nakhoul in Beirut and John Irish in Paris; Writing by Angus MacSwan; Editing by Giles Elgood