A huge car bomb explosion in Beirut today killed a top Lebanese security official and seven other people.
The rush-hour bomb in the centre of the Lebanese capital wounded about 80 others, heightening fears that Syria's war is spilling over into Lebanon.
Among the dead was Wissam al-Hassan, whose investigations implicated Syria and Hezbollah in the assassination of former Lebanese prime minister Rafik al-Hariri seven years ago.
Mr Hassan was 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.
Mr Hassan was a close aide to Mr Hariri, a Sunni Muslim who was killed in a 2005 bomb attack in downtown Beirut.
Sunni Muslims took to the streets and burned tyres across Lebanon in protest against the killing of Mr Hassan.
Protesters blocked the streets in Sunni strongholds of the eastern Bekaa valley region, the northern area of Akkar, neighbourhoods of the capital Beirut and in the southern city of Sidon.
Today’s bombing was seen as being linked to the heightened tension between Lebanese factions on opposite sides of the conflict in Syria.
The explosion 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 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," he said.
His own brother was assassinated in November 2006.
The war in Syria, which has killed 30,000 people in the past 19 months, has pitted mostly Sunni insurgents against Assad, who is from the Alawite sect linked to Shia Islam.
Lebanon's religious communities are divided between those supporting Mr Assad and those backing the rebels trying to overthrow him.
The blast occurred when parents were picking up children from school, and sent black smoke billowing into the sky.
Several cars were destroyed and the front of a multi-storey building was badly damaged, with tangled wires and metal railings crashing to the ground.
Lebanese Prime Minister Najib Mikati said the government was trying to find out who carried out the attack and those responsible would be punished.