A powerful bomb has killed a senior Lebanese police intelligence officer and three others in east Beirut.
Capt Wissam Eid had been leading inquiries into a string of bombings that have shaken Lebanon since 2004.
His convoy was blown apart as it passed an explosives-laden vehicle detonated by remote control near a major highway.
Crisis-hit Lebanon has been unable to elect a president amid rivalry between the Western-backed government and the opposition supported by Syria and Iran.
Bomb attacks over the last three years have targeted prominent members of the anti-Syrian movement, but since last month they have also struck an army chief and a US embassy vehicle.
The blast was heard throughout central Beirut and sent a thick plume of black smoke over eastern, mainly Christian suburbs.
TV pictures quickly showed what appeared to be the aftermath of a large blast causing damage to dozens of cars.
Local people, some shouting others in shock, could be seen running through the tangled and burning vehicles.
Precise casualty figures were not immediately available because wreckage and human remains were scattered over a wide area.
One charred body was trapped in a car and another was hurled up to a nearby overpass.
Security officials said the bomb, which left a crater five metres (16 feet) wide, had been made with at least 50 kg (110 pounds) of explosives.
Thirty-eight people were injured in the explosion, police said.
Capt Eid, of the Internal Security Forces, had been targeted before, in a grenade attack on his home. He took up his post after his predecessor, Samir Shehadeh, was wounded in a roadside bomb, officials said.
The blast comes at a time of acute political instability in Lebanon as deadlock between pro-Syrian and pro-Western parties has prevented the election of a president for more than two months.
Last year the country was also rocked by an uprising by Islamist militants in the north and deadly attacks on UN peacekeepers in the south.
"This is a message to the Internal Security Forces, following the message sent to the army in December when General Francois el-Hajj was killed in a car bomb," said ISF chief Ashraf Rifi.
Lebanon's parliamentary majority leader Saad Hariri indirectly accused Syria of involvement, in a statement which he urged Lebanon's more powerful larger neighbour to stop its "interference" and "score-settling".
"This attack is a clear message to all Arabs that the future of Lebanon will remain under the stranglehold of crimes and terrorism," said the son and political heir of Rafik Hariri, who was killed in a car bombing in February 2005.
Damascus, which denies any involvement, condemned the latest attack, saying it "targeted Lebanon's security and stability".