A Lebanese journalist who championed a years-long crusade against Syrian influence in Lebanon through both writing and activism was assassinated Thursday by a bomb placed in his car near his home in the capital. The attack, which his supporters and some Lebanese politicians blamed on Syria and its lingering presence here, elicited cries of outrage throughout the country.

Samir Kassir, 45, was the most prominent Lebanese figure killed since Rafiq Hariri, a former prime minister, died in a bombing on Feb. 14. Hariri's slaying unleashed mass protests in downtown Beirut that led to the end of the 29-year Syrian military presence in the country.

Kassir, a columnist for Lebanon's An Nahar daily, helped shape the message of the spring protests, dubbed the Cedar Revolution.

In an interview with The Washington Post last week, Kassir celebrated the movement's accomplishments, even as he spoke soberly about what was ahead.

"People are tending to forget that we have tremendous change in Lebanon," he said from his office overlooking Martyrs' Square. "What's at stake now is to show that an open political game is now available in Lebanon."

Bowing to international pressure, Syria withdrew its troops in April, but it maintains influence in the small Mediterranean country, particularly with the intelligence services and political allies, including President Emile Lahoud.

The United States and France denounced the killing, as did U.N. Secretary General Kofi Annan, who lamented the death of a "prominent and outspoken" journalist. In Washington, Secretary of State Condoleezza Rice called the assassination "a heinous act."

In Lebanon, opposition leaders demanded the resignation of Lahoud, whose standing has eroded dramatically since the Syrian troop pullout, and called for a general strike on Friday to protest the killing. Anti-Syrian politicians had declared Lahoud's ouster to be their goal once a new parliament is chosen in elections that end June 19.

"The response to this new crime should be the resignation of the president as the head of the security and intelligence regime," said a statement issued Thursday night by opposition leaders after a meeting in the capital.

Syria denied any role in the assassination. Its official SANA news agency quoted an Information Ministry official as saying the allegations were designed to increase pressure on Syria.

Lahoud, who quickly condemned the killing, visited the headquarters of the Lebanese journalists union to pay his respects and convened an emergency cabinet meeting to discuss security.

Kassir was killed when a small bomb exploded in midmorning after he got into his silver Alfa Romeo near his house in the fashionable Christian neighborhood of Ashrafiyyeh. The blast set the car on fire, twisted its frame and shattered windows, strewing glass along the sidewalk. The Interior Ministry said the explosion, which came from under the driver's seat, killed him instantly. His mangled torso was spread across the passenger's seat before rescue workers moved his corpse onto a stretcher.

Visibly angry crowds gathered around the scene, which was cordoned off by security forces and soldiers.

"This country can't go back to normal if the security agencies aren't purged, and before Emile Lahoud personally resigns," said Elias Atallah, the leader of a small leftist party and a close friend of Kassir's, who wept as he surveyed the scene. "Those relations with Syria were never interrupted. Every day they visit them and they coordinate with them," he added, referring to Lebanese officials.

Kassir had for years called for an end to Syria's role in Lebanon, criticizing the authoritarian government in Damascus and what he called the Lebanese police state. Given Syria's intimidating influence, the stand was considered courageous. His passport was seized in 2001 and he was threatened with arrest.

Kassir, a former communist with a trimmed, graying beard, was a charming figure recognizable to many of Beirut's elite, who were devoted to his combative weekly column on An Nahar's front page. He combined a reporter's fondness for gossip with a piercing intellect and world-weary wit. He spread criticism freely: Lebanese politicians, Hezbollah, his own paper and the Syrian government, which he derided as a mafia.

During the interview last week, he was dragging on a Gauloise cigarette, as usual. Newspapers were stacked on his desk, with the inspiration for his next column.

Given Kassir's profile, the attack reverberated across Lebanon and the region, where Lebanese and Arab satellite networks broadcast footage of the attack and segments of interviews with Kassir, who also lectured at St. Joseph University in Beirut.

"This is absolutely not acceptable. We shall not relent until we find those who commit such acts," said Saad Hariri, the 35-year-old son of the slain former prime minister and a candidate to lead Lebanon's next government. In remarks to journalists, he compared the killing to his father's, which many also blamed on Syria. "God knows what's coming next," he said.

The killing was the starkest reminder yet of Lebanon's tenuous political landscape, even after the withdrawal of Syrian troops. Since their departure, the anti-Syrian opposition has splintered, with one camp of Lebanon's traditional political leadership rallying around Hariri, the other loyal to a Christian former general, Michel Aoun, who returned in May after 15 years of exile in France. Disenchantment with the old political class is rife, and fears still run deep over Syria's intentions.

The country began voting Sunday for a new parliament, the first election in three decades without a Syrian military presence. The elections will continue for three successive Sundays, with voting in southern Lebanon this week.

"Each time we take a step forward, something happens to take us a step back," Prime Minister Najib Mikati said at the bombing scene.

Friends said Kassir, who was married to a well-known television journalist, had received death threats. In interviews, he was blunt about the threat he saw to his safety. For a time in 2001, dozens of intelligence agents unabashedly followed his movements.

"Samir lived all his life in danger," said his brother, Suleiman Kassir.

Jibran Tueini, the publisher of An Nahar and a newly elected member of parliament, urged journalists and others not to be intimidated by the killing. Though riddled by influence-peddling, Lebanon's press remains one of the region's most unencumbered.

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

Special correspondent Alia Ibrahim contributed to this report.

Samir Kassir wrote for Lebanon's An Nahar daily. A woman cries as she passes the scene of the explosion that killed anti-Syrian columnist and activist Samir Kassir, 45.