The opposition "demands by democratic means the resignation of the president as he is the effective leader of the security/intelligence regime", Elias Atallah, a senior official in the Democratic Left movement, said.
"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," Atallah said, reading a statement after an emergency meeting of the opposition on Thursday in Beirut.
Kassir, who wrote anti-Syrian articles for Lebanon's leading An-Nahar newspaper, was killed when a bomb planted under his car exploded in a neighbourhood of mainly Christian east Beirut earlier on Thursday.
Hundreds of Lebanese journalists, opposition supporters and relatives of Kassir gathered at the Martyr's Square in downtown Beirut on Friday to protest against his assassination.
Kassir was killed in a car bomb
blast in Beirut on Thursday
They stood in silence to mourn Kassir, Aljazeera's correspondent in Beirut reported.
The silent protest was organised by the An-Nahar newspaper.
Aljazeera also reported that participants in the Bristol meeting, representing Lebanese opposition factions, called on UN chief Kofi Annan to entrust the UN commission that certified the pullout of Syrian forces in April to return to verify the withdrawal of all Syrian intelligence agents.
The opposition gathering also called for a nationwide general strike on Friday and for massive participation in a demonstration on Monday to denounce the murder of "a pillar of Arab democracy and one of the heroes of the uprising for the independence from Syrian domination".
The opposition asked the UN to
again verify Syrian withdrawal
The Bristol meeting described the response to Kassir's killing as the continuation of Lebanon's "independence uprising", Aljazeera said.
The opposition said: "This great political murder is a new cycle in the plot to destroy the nation, and which started with the assassination of Rafik al-Hariri and (MP) Bassel Fleyhan."
They said the assassination came after "the Lebanese-Syrian security regime ... thought that the uprising had achieved its objectives ... and this allowed them to take a new breath in order to again attack the opposition and its symbols".
For his part, President Lahoud was quoted by Aljazeera as cautioning against internal strife in the wake of Kassir's killing.
The closed-door opposition meeting was attended by al-Hariri's son Saad al-Hariri, influential Druze MP Walid Jumblatt as well as leading Christian figures and Jibran Bassil, son-in-law of former exiled Christian hardliner Michel Aoun.
But Bassil walked out before the end of the meeting in protest against "the exploitation of the crime for political ends" in demanding Lahoud's resignation.
"The security services are now under the control of al-Hariri followers and should assume their responsibility in uncovering the criminals"
son-in-law of Michel Aoun
"The security services are now under the control of al-Hariri followers and should assume their responsibility in uncovering the criminals," he said, referring to the ministers of interior and justice who are close to the al-Hariri family.
But Atallah accused Aoun of breaking opposition ranks since his return from 15 years of exile in France for waging a failed "war of liberation" against the Syrians in Lebanon.
"Today, he is forging alliances with the symbols of the agonising pro-Syrian regime," he said, referring to ongoing political negotiations for the ongoing legislative elections.