Tunis, Tunisia (CNN) -- Opposition leader Chokri Belaid routinely received death threats for his outspoken criticism of Tunisia's Islamist-led government. He talked about the bullying on his frequent television appearances but said he didn't fear for his life.
But on Wednesday morning, a gunman shot and killed Belaid in front of his home as he was leaving for work, according to an eyewitness. Belaid's political allies and even his Islamist opponents are decrying the murder as an assassination, sparking new protests in the cradle of the "Arab Spring."
The killing of Belaid, 48, has shocked the nation. Police had fired on protesters, killing some of them, during the December 2010-January 2011 revolt that drove the country's longtime strongman into exile. But the targeted slaying of a politician crossed a new line.
Video showed outraged protesters filling the Avenue Habib Bourguiba in Tunis and pouring into the streets of other Tunisian cities. Some of them revived the iconic slogan of their revolution, crying: "The people want to topple the regime."
Angry clashes broke out in front of Tunisia's Interior Ministry, where police used tear gas to disperse the protesters. Sympathizers not interested in politics also turned out to mourn the popular public figure and express their shock about the violence that has marred the country's heated -- but otherwise previously peaceful -- public debate.
Prime Minister Hamadi Jebali, himself an Islamist and head of the moderate religious party Ennahda, quickly condemned the bloodletting on state television. By Wednesday night, Jebali had sacked his Cabinet and called for new elections, leaving himself at the head of a caretaker government.
"Belaid was killed, but the real target behind the assassination is the Tunisian revolution as a whole,"Jebali said of his political adversary. "He represented the true values of dialogue, respecting and embracing others in rejecting violence. This is a political assassination."
But his expression of indignation did not stop angry protesters from storming Ennhada offices. Belaid's brother, Abdelmajid Belaid, blamed Jebali's Ennhada party for the killing.
Belaid's death was "a clear message to Tunisians ... shut up, or we kill you," Abdelmajid Belaid said. He said his brother "has been been receiving threats of murder for a long time," including a text message Tuesday.
And Chokri's widow, Basma, told Tunisian state TV: "We are damned. The political struggle is damned in Tunisia. Chokri Belaid sacrificed his soul."
Belaid's support went beyond his own party, the secular-leftist Democratic Patriots. He was the voice of a large coalition of secular opposition parties known as the Popular Front and had a reputation for decrying violence.
Interior Minister Ali al-Areed vowed to track down the killers and joined the chorus of moral indignation, calling Belaid's killing "an attack on all Tunisians." At the same time, he asked that protests remain peaceful: "We do not want the country to fall into chaos," he said on Tunisian state TV.
French President Francois Hollande also condemned the assassination, saying Belaid's murder deprived Tunisia "of one of its most courageous and free voices."
Belaid had fought throughout his public life for liberty, tolerance and respect for human rights, Hollande said, with the deeply held conviction that dialogue and democracy must be at the heart of the new Tunisia.
Hollande said France, Tunisia's onetime colonial ruler, is concerned about the increase in political violence. He urged respect for the ideals that drove the country's revolution.
And in Washington, U.S. State Department spokeswoman Victoria Nuland called for Tunisia's government "to conduct a fair, transparent and professional investigation" of Belaid's killing.
"It's also important to note that a broad base of Tunisian political parties and actors have also condemned this violent act," Nuland said. "There is no justification for an outrageous and cowardly act of violence such as this. There is no place in the new Tunisia for violence."
One of Belaid's neighbors heard the first shot discharge in the quiet Tunis suburb of Menzah 6. He looked out of his apartment to see a gunman fire two more shots at close range.
At least one bullet hit Belaid squarely in the chest, said Dr. Hedi Tebourbi. The physician felt certain he must have died right away.
A man waited close by on a motorbike for the gunman to hop on, Tebourbi said. They sped away from the scene.
An ambulance took Belaid to a nearby hospital, where doctors pronounced him dead.
Journalist Houda Zaghdoudi reported from Tunis. CNN's Saad Abedine and Ben Brumfield reported from Atlanta, and Laura Smith-Spark from London.