Lebanon's head of internal intelligence has been killed in a massive car bomb attack in central Beirut.
Wissam al-Hassan was among eight people who died in the attack. He was close to opposition leader Saad Hariri, a leading critic of the government in neighbouring Syria.
Dozens were wounded in the blast, which Mr Hariri blamed on Damascus. Syria's government condemned the bombing.
Tension in Lebanon has been rising as a result of the Syrian conflict.
Mr Hassan, the head of intelligence of Lebanon's internal security forces, was regarded as an opponent of Syria.
He led an investigation that implicated Damascus in the 2005 bombing that killed Mr Hariri's father, former Prime Minister Rafik Hariri.
He also recently organised the arrest of a former minister accused of planning a Syrian-sponsored bombing campaign in Lebanon.
Friday's attack, the deadliest in Beirut since 2008, occurred in the mainly Christian district of Ashrafiya, in a busy street close to the headquarters of Saad Hariri's 14 March coalition.
Lebanon's Shia militant group Hezbollah - a close ally of the Syrian government - condemned the bombing.
Syrian Information Minister Omran al-Zoubi called it a "cowardly terrorist act".
Mr Hariri accused Mr Assad of being behind the bombing - an accusation echoed by Druze leader Walid Jumblatt, a longtime critic of Damascus.
The 14 March bloc issued a statement accusing the Beirut government of protecting "criminals" and calling on it to stand down.
Opposition supporters took to the streets in several cities, burning tyres and denouncing the Syrian authorities and Hezbollah.
Lebanese Prime Minister Najib Mikati said the government was trying to identify the perpetrators and they would be punished.
Correspondents say Friday's attack was reminiscent of scenes from Lebanon's civil war in the late 1970s and 1980s.
The blast, which was heard several kilometres away, set many cars ablaze and destroyed the facades of nearby buildings.
Ronnie Chatah, who lives nearby, told the BBC: "The building shook and it echoed throughout the neighbourhood."
About 80 people have been wounded. Hospitals across the city called for people to donate blood.
In Washington, the state department condemned the bombing.
Spokeswoman Victoria Nuland said the US would "stand by the people of Lebanon" and reaffirmed her administration's "commitment to a stable, sovereign and independent Lebanon".
The United Nations Security Council also issued an "unequivocal condemnation".