Gibran Tueni, a staunchly anti-Syrian MP and journalist, was killed along with three others when a car bomb blew up his armoured vehicle in the mainly Christian suburb of Mekalis in east Beirut at around 9am (0700 GMT) on Monday morning.
Director of Lebanese newspaper An-Nahar, Mr Tueni was one of the most vocal campaigners against Syrian interference Lebanon.
Lebanon's President Emile Lahoud described Mr Tueni as "the country's symbol of freedom", adding that the MP had joined "the motorcade of martyrs led by former prime minister Hariri... his death only serves the enemies of Lebanon."
He added: "Lebanon has lost a prominent figure who has devoted his life to support freedom of (the) press and freedom of expression."
Syria claimed the attack was part of a larger plan to implicate Damascus and damage its reputation.
"Syria considers this crime as part of a larger vicious plan attempting to implicate Syria and cause the maximum possible damage to its reputation at a particularly crucial time," a statement issued by the Syrian embassy in
Washington said.
The statement condemned the killing of Tueni "as yet another crime targeting prominent Lebanese leaders and trying to undermine the security and stability of Lebanon".
International reaction
The White House strongly condemned the slaying and demanded enforcement of UN Security Council resolutions aimed at ending Syrian interference in Lebanon.
But White House spokesman Scott McClellan told reporters that it was "premature to know exactly who is responsible" for the powerful car-bomb blast.
"I think that his murder is another act of terrorism aimed at trying to subjugate Lebanon to Syrian domination," he said.
"It's clear this savage attack is clearly intended to intimidate those in Lebanon who would courageously and openly speak their minds," Mr McClellan said.
"The attack is a reminder that all of us in the international community must continue to insist on enforcing the Security Council resolutions aimed at ending Syria's interference in Lebanon once and for all," he added.
The European Union also condemned the attack, calling it a crude act of political violence.
"The EU is deeply concerned at this latest in a series of attacks against supporters of Lebanese democracy," said British Foreign Secretary Jack Straw, whose country holds the rotating EU presidency.
The UN Security Council was due to hold impromptu consultations on the Tueni slaying.
"The secretary-general strongly condemns this cold-blooded murder," a UN statement said, adding that "the perpetrators and instigators … must be brought to justice."
Lebanese Prime Minister Fuad Saniora said he will ask the UN to set up a new inquiry into Tueni's slaying and previous bombings and to create an international tribunal to try suspects in the Hariri assassination.
Syria implicated
Monday's attack came just hours before the UN Security Council received the latest report on a probe into the murder of Hariri in a similar car bomb attack in Beirut in February, which has been blamed on Syria.
The report, by UN chief investigator Detlev Mehlis, cites new evidence further implicating senior Syrian officers.
It followed the release of an interim report by the Mehlis team in October which found evidence implicating senior Syrian and Lebanese security officials.
Damascus has insisted it has been falsely accused of orchestrating the killing and has pledged to cooperate fully with the UN enquiry panel.
"In late 0ctober 2005, the commission was approached by another new witness, who has submitted a comprehensive and coherent statement regarding plans to assassinate Mr Hariri," the latest report said. "The witness has been assessed to be credible and the information he has submitted to be reliable."
"The statement from the witness strengthens the evidence confirmed to date against the Lebanese officers in custody, as well as high-ranked Syrian officers," it added.
The report also raised questions about Syria's commitment to get to the bottom of the Hariri slaying through its own judicial investigation.
Syria denies involvement and has waged a campaign to discredit the commission, citing a Syrian witness, Husam Taher Husam, who recanted his testimony to the commission and said he had been bribed to frame Syria.
Mr Mehlis said that recantation hadn't affected his findings.
As a result of the October report, the Security Council unanimously adopted Resolution 1636 demanding Syria fully cooperate with the probe or face international sanctions.
Lebanon has asked the Security Council to extend Mehlis' commission for six months after its mandate expires on Thursday.