Samir Kassir of the An-Nahar newspaper was killed instantly when a bomb exploded in his car as he started the engine outside his home in the Ashrafiyeh neighbourhood.
A woman accompanying him was wounded. Her identity was not immediately known.
Police, who cordoned off the area around the vehicle, said the bomb was placed under the driver's seat.
Kassir was a columnist at Lebanon's leading daily who for years had called for an end to Syria's role in Lebanon.
Syria ended its 29-year military presence in Lebanon in April under international and Lebanese popular pressure.
The blast came four days after the start of the country's parliamentary elections, staggered over four Sundays from 29 May to 19 June.
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," said Walid Jumblatt, a vocal opponent of Syria.
Lahoud's spokesman Rafik Shalala described the murder as a "grave incident".
Prime Minister Najib Mikati and Interior Minister Hassan Sabei arrived after emergency services at the scene.
"Every time Lebanon takes a step forward, there are those who want to undermine this country," Mikati said.
He called the murder painful and ordered security agencies to take measures to uncover the circumstances of the bombing.
"Samir Kassir was assassinated by the remnants of the security agencies that control the country and that is headed by Emile Lahoud"
"We will not allow anyone to target security and freedom," Mikati said.
Michel Aoun, an opposition leader who was announcing his list of candidates for the elections in central Lebanon, began the news conference with a moment of silence, describing Kassir as "one of the great journalists who never hesitated or was afraid to say the truth and defend Lebanon".
Gibran Tueni, An-Nahar's general manager, linked the killing to the 14 February bombing that killed former Prime Minister Rafiq al-Hariri and 20 other people and to another explosion targeting an opposition lawmaker who escaped with serious injuries.
"The Lebanese security authorities and the remnants of the Syrian system in Lebanon, and directly the Syrian regime from top to bottom, is responsible for every crime and every drop of blood spilled," Tueni said at the scene.
Arab journalists group
The Arab Committee for the Defence of Journalists on Thursday expressed its regret for what it called "the terrorist act which caused the death of the distinguished Lebanese writer and journalist Samir Kassir, after exploding his car in front of his house in Al- Ashrafiyah district in Beirut today".
Many Lebanese politicians blamed
the pro-Syrian government
In a news release received by Aljazeera.net, the committee called for an immediate investigation.
Lebanon is in the midst of parliamentary elections that
began on 29 May and run through 19 June. Tueni won a seat from Beirut for the opposition last Sunday.