Prominent Lebanese anti-Syrian MP and journalist Gibran Tueni has been killed in a massive car bomb attack in Beirut.
The attack happened on a mountain road in East Beirut
It came hours before UN Security Council members got the latest report on the death of a former Lebanese PM, in which Syria has been implicated.
The report, by German investigator Detlev Mehlis, urges Damascus to arrest Syrian suspects, Reuters agency says.
UN Secretary General Kofi Annan condemned Monday's "cold-blooded murder" in a statement.
Syria had said the attack was timed to damage its reputation ahead of a UN meeting.
BBC Middle East analyst Roger Hardy says the attack raises the political temperature in Lebanon at a highly sensitive moment.
Mr Tueni was killed only a day after returning from Paris, where he had been staying amid fears for his life.
A previously-unknown group calling itself Strugglers for the Unity and Freedom of the Levant issued a statement saying it had carried out the attack. There was no independent confirmation of the group's claim.
There were emotional scenes as news of the attack spread.
One elderly man wept openly at the bomb site, shouting: "My God, Gibran, you were the only one who told the truth!" AP news agency reported.
Mr Tueni's car was travelling through the Christian-dominated Mekallis area of eastern Beirut during the morning rush hour when the attack happened.
The blast was so powerful it blew his armoured car off the small mountain road and sent it rolling down the hill, the BBC's Kim Ghattas reports from Beirut.
The body of Mr Tueni and three others were recovered several hundred metres away from the blast site. At least 10 people were wounded.
'Voice of freedom'
Mr Tueni was managing editor of the leading liberal An-Nahar newspaper, and an outspoken critic of Syria's occupation of Lebanon.
Gibran Tueni was a vocal critic of Syria's occupation of Lebanon
The 48-year-old - who had recently remarried and was the father of twins - was elected to parliament in June.
Mr Jumblatt accused Syria of targeting Mr Tueni because "he was the voice of freedom".
"He was upsetting directly the Syrian regime. He was frank about it," the Druze leader told the BBC.
"He was saying to the Syrian people, to the Arab world, these kinds of regimes, terrorist regimes, cannot still exist, they should disappear."
But Damascus vehemently denied any involvement.
The attack's "timing is intended to direct accusations to Syria", Damascus said in a statement carried by the official Syrian news agency.
"Syria denounces this crime that claimed the lives of Lebanese, irrespective of their political stances," Syrian Information Minister Mahdi Dakhl-Allah told LBC television.
The US called the murder of Mr Tueni a "heinous act".
"The forces of oppression and tyranny have taken from the Lebanese people one of their greatest champions for liberty and freedom and have killed a courageous advocate of Lebanese independence and sovereignty," the US embassy said in a statement.
French Foreign Minister Philippe Douste-Blazy said the attack had been carried out by those trying to destabilise Lebanon.
"The international community stands united and determined at Lebanon's side during this crucial time for its future," he said.
A second report by investigator Detlev Mehlis into the killing of ex-prime minister Rafik Hariri was handed to UN Secretary General Kofi Annan on Sunday.
Copies are due to be handed to members of the UN Security Council for discussion at a meeting on Tuesday.
Damascus has strongly denied any involvement in the Hariri killing, but an interim report by Mr Mehlis' team has already implicated Syrian officials.
The massive car bomb attack on Mr Hariri's convoy in February led to huge anti-Syria protests in Lebanon, and the withdrawal of Syrian troops.
Since the killing of Mr Hariri, Christian and anti-Syrian areas have been the target of at least 14 blasts, says our correspondent.
Another An-Nahar journalist, the anti-Syrian writer Samir Kassir, was killed in a car bomb in June.