Samir Kassir, whose newspaper column regularly criticised the country’s pro-Syrian government, was killed when a bomb exploded under his car on Thursday.
His death, four days after the start of Lebanon’s staggered parliamentary elections, shocked a country that is coming to terms with the assassination of former Prime Minister Rafik al-Hariri in February.
As thousands joined a candlelit vigil for Mr Kassir in Beirut, the opposition called for Lebanese President Emile Lahoud to resign over the bombing.
“Once again the hand of terrorism, under the protection of the president and the joint Lebanese-Syrian intelligence agencies and what is left of the regime, targets a symbol of the free press,” said an opposition statement.
President Lahoud condemned the killing, saying it targeted the unity of the Lebanese people.
Opposition politicians have also pointed the finger at Damascus.
“The Syrian regime is responsible from head to toe for this horrific terrorist crime,” said Jibran Tueni, a politician from An-Nahar, who won a seat in Beirut’s polls last Sunday.
Damascus angrily rejected accusations it had any role in Mr Kassir’s death.
The US Secretary of State Condoleezza Rice described Mr Kassir’s killing as a “henious act”.
The European Union also condemned his murder, with France saying it “strongly condemned (the assassination of) one of the symbols of the freedom and independence of the Lebanese press.”
The journalist’s death has reignited anti-Syrian feelings in Lebanon stirred up after the assassination of former premier Rafiq Hariri.
Mr Hariri’s murder triggered protests which helped pave the way for the withdrawal of Syrian troops from Lebanon, a move that was completed in April under the weight of intense international pressure.
A Christian of Palestinian origin who held Lebanese and French nationality, Mr Kassir was an editorial writer for the leading An-Nahar newspaper.
He regularly wrote virulent articles against the Lebanese regime and maintained close links with the Syrian opposition.
In his last editorial Mr Kassir – whose passport was confiscated in 2000 following articles against the “police regime” in Lebanon – criticised the detention of political activists in Syria.
“The Baathist regime in Syria is behaving the way it behaved in Lebanon, making blunder after blunder,” he wrote.
General elections are under way in Lebanon staggered over four Sundays from May 29 to June 19.