A prominent Lebanese journalist and political activist who campaigned against Syrian influence in Lebanon was assassinated Thursday by a bomb placed in his car, an attack that elicited cries of outrage in the country and that his supporters and some Lebanese politicians blamed on Syria and its lingering presence here.

Samir Kassir, 45, an influential columnist for the Lebanese daily An Nahar and a coordinator of Lebanon's political opposition movement, was killed when he started his white Alfa Romeo outside his home in Beirut's predominantly Christian Ashrafiyeh neighborhood. Police told reporters that the bomb was placed under the driver's seat and tore apart Kassir's lower body, setting the car on fire. The blast also wounded a passerby, damaged several cars and shattered windows in nearby buildings.

Lebanese opposition leaders denounced the killing and said they would press for the resignation of President Emile Lahoud, who has supported the Syrian presence in Lebanon. After a meeting Thursday night, the opposition called for a general strike Friday and a day of national mourning. The opposition leaders urged Lebanese to join a massive funeral march, but said they would not allow any government participation in the rites for Kassir.

"This major crime is a new episode in plans to destroy the nation," opposition spokesman Elias Atallah said. He said efforts to force Lahoud's resignation would be "pursued with all the democratic and constitutional methods available and by a referendum on eliminating this presumptuous regime."

The U.S. ambassador to Lebanon, Jeffrey D. Feltman, called Kassir a symbol of the Lebanese desire for freedom. He said he does not know who was responsible for the bombing but suggested that it was part of a pattern of recent political assassinations.

The bombing came four days after the start of parliamentary elections in which opponents of Lahoud's Syrian-backed government scored an opening-round victory, sweeping all 19 seats up for grabs in Beirut. The elections were scheduled to be held in different parts of the country on four Sundays from May 29 to June 19.

The polls are the first to be held in Lebanon since neighboring Syria withdrew its troops under international pressure in April, ending a 29-year military presence that had aroused growing public resentment. Calls for Syrian withdrawal intensified after former prime minister Rafiq Hariri, a leading opponent of the Syrian presence, was assassinated by a massive car bomb that blew up his motorcade along the Beirut waterfront on Feb. 14. Hariri's 35-year-old son, Saad Hariri, headed the ticket that captured Beirut's 19 parliamentary seats Sunday in the first of the election's four stages.

Kassir, who helped promote huge street demonstrations after Hariri's assassination, had frequently run afoul of the Lebanese and Syrian governments for his weekly front-page columns, which regularly criticized the Syrian presence in Lebanon and won him a devoted following.

Saad Hariri called the killing of Kassir "absolutely not acceptable" and vowed, "We shall not relent until we find those who commit such acts." He likened the assassination to the bombing that killed his father and to an unsuccessful attempt last fall on the life of Marwan Hamadi, a former Druze cabinet minister. "God knows what's coming next," Hariri told reporters somberly as he emerged from a meeting with other opposition leaders.

"We will not stop," he said in reference to the country's attempt to elect a new parliament despite low voter turnout. "We want our freedom. We want our sovereignty, and no one is going to stop us."

Although there was no claim of responsibility for the bombing and no immediate evidence pointing to a specific perpetrator, some Lebanese opposition figures and friends of Kassir were quick to assign blame.

"Samir Kassir was assassinated by the remnants of the security agencies that control the country and that is headed by Emile Lahoud," opposition leader Walid Jumblatt told Future television, an opposition outlet, the Associated Press reported. Jumblatt, who heads a Druze political party, called for the ouster of Lahoud, who strongly supported the Syrian presence.

"As long as . . . the head of the snake is in Baabda, the assassinations will continue," he told al-Arabiya television, referring to the Lebanese presidential palace in the Beirut suburb of Baabda.

Lahoud condemned the assassination of Kassir and called an emergency cabinet meeting. He said the killing was directed against the unity of the Lebanese people, and he visited the headquarters of the Lebanese journalists' union to pay his respects.

Gibran Tueni, the general manager of An Nahar and winner of a parliamentary seat in Sunday's voting, said at the scene of the bombing, "The Syrian regime is responsible from head to toe for this horrific terrorist crime. Lebanon's opposition should promptly close ranks anew to have every Syrian intelligence cell left behind in Lebanon ruthlessly smashed." He said Syrian President Bashar Assad "should not be allowed to have a single intelligence operative lingering in Lebanon."

Nasib Lahoud, an opposition member of parliament, also spoke to reporters at the scene, calling Kassir "one of the leaders of Lebanon's spring." Nasib Lahoud said, "The battle with intelligence apparatus in Lebanon is not over." The elections now underway "will hopefully put the majority in parliament in our hands and will allow us to take control of government," he said. "Then an effective way of turning Lebanon in a new direction would be much more possible."

Although Syria has withdrawn its troops, it is widely believed to maintain a discreet presence in the small, Mediterranean country, particularly among Lebanese intelligence services and political allies, led by President Lahoud.

France, with historic ties to Lebanon, denounced the killing, and opposition leaders issued calls to unite their divided ranks. The assassination may escalate pressure on Lahoud, whose standing has dramatically eroded since Syria withdrew, political analysts said.

There was no immediate statement from the Syrian government.

After the bombing, visibly angry crowds gathered around the site, which was cordoned off by police and soldiers.

Parents rushed to pick up their distraught children from a nearby elementary school, and shouting matches erupted between civilians with opposing political views, said washingtonpost.com videographer Pierre Kattar, who arrived at the scene shortly after the blast.

Some people wept as they watched from balconies overlooking the bombing site, Kattar said.

In his columns, Kassir for years had called for an end to Syria's role in Lebanon, criticizing the Syrian government and what he called the Lebanese police state. Given Syria's influence, the stand was considered courageous. His passport was seized in 2001 after an especially critical series of articles, and Lebanese intelligence agents followed him for more than a month. He was threatened with arrest, and friends said he received death threats.

In addition to writing for An Nahar, the bearded, chain-smoking Kassir was a political science lecturer at St. Joseph University, a Jesuit school in Beirut.

In an interview in April, Kassir told The Washington Post, "Many of us have been writing for years about Syria's mafia-like presence in Lebanon, and the fact that the Syrian army is a much less malign influence here than their intelligence services."

Suleiman Kassir, the late columnist's brother, confirmed that he had received death threats, adding, "Samir lived all his life in danger."

Tueni told reporters, "Our battle is not yet over with the Syrian-Lebanese security regime. Had it not been for the free press in Lebanon, this country would not have been able to reclaim its freedom." He added, "Our message is more powerful than any bomb. Do not be afraid."

Interior Minister Hasan Sabaa said the killing was intended to "scuttle all that the Lebanese people had managed to accomplish in recent months" and amounted to "a desperate attempt to take Lebanon hostage and to block the road toward democracy."

Prime Minister Najib Mikati, who was named to the post after his predecessor stepped down in March in the face of street protests, said: "Each time we take a step forward, something happens to take us a step back."

Hamadi, who still walks with difficulty because of injuries he suffered in last fall's assassination attempt, said the murder of Kassir "is a new crime by the Syrian-Lebanese regime."

He called on Lebanese residents to press for the ouster of Lahoud by taking to the streets as they did in protest against the assassination of Hariri.

Branigin reported from Washington.