Anti-Syrian groups quickly blamed Damascus and its Lebanese allies, especially President Emile Lahoud, Syria's greatest supporter in Lebanon.
"Samir Kassir was assassinated by the remnants of the security agencies that control the country and that is headed by Emile Lahoud," Walid Jumblatt, a vocal opponent of Syria, said on Future television.
"As long as the serpent's head is in Baabda, the assassinations will continue," he told Al-Arabiya television, referring to the presidential palace in the Beirut suburb of Baabda.
Lahoud's spokesman Rafik Shalala described as the murder as a "grave incident."
The explosion occurred midmorning in a Christian neighborhood of Beirut, setting the car on fire and killing the occupant. Police, who cordoned off the area around the vehicle, said the bomb was placed under the driver's seat.
Kassir's body lay slumped on one side inside the blasted sedan.
Kassir wrote a column in An-Nahar, the leading newspaper that frequently criticizes Syria and the former Lebanese government that was allied with Damascus.
Prime Minister Najib Mikati and Interior Minister Hassan Sabei went to the scene.
"Every time Lebanon takes a step forward, there are those who want to undermine this country," Mikati said. Calling the murder painful, he ordered security agencies to take measures to uncover the circumstances of the bombing.
"We will not allow anyone to target security and freedom," Mikati said.
Neighborhood residents said the explosion occurred after Kassir, who lived across the street, got into his car. The blast shattered the windows of some nearby apartment buildings.
"I heard a loud explosion. I was thrown out of my place. People rushed and there was a car burning," said Manuel Izmizilian, who works at a nearby optician's shop.