Prominent Tunisian opposition leader Chokri Belaid was gunned down outside his home in Tunis on Wednesday, sparking angry protests by his supporters and attacks on offices of the ruling Islamist Ennahda party.
Furious protesters built barricades in central Tunis and clashed with police who fired tear gas to disperse them, and four opposition groups including Belaid's Popular Front bloc said they were pulling out of the national assembly.
President Moncef Marzouki denounced the killing of Belaid, an outspoken critic of his government, as an "odious assassination," while Ennahda chief Rached Ghannouchi told AFP the killers wanted a "bloodbath."
The US embassy issued a statement calling the killing an "outrageous and cowardly act."
The murder sparked outrage, with violence reminiscent of the uprising that ousted veteran dictator Zine El Abidine Ben Ali just over two years ago, and thousands rallying outside the interior ministry.
The four opposition parties demanded Interior Minister Ali Laraydeh's sacking, blaming him for Belaid's murder "because he knew he was threatened and he did nothing," said Nejib Chebbi, leader of one of the groups.
The family of Belaid, who headed the Party of Democratic Patriots which is part of the Popular Front, accused Laraydeh's Ennahda of being behind the assassination.
The wife of the 48-year-old leftist leader said her husband had received daily death threats and was murdered before her eyes.
"I saw his blood flowing, I saw his little smile. I saw that they want to kill democracy," Basma Belaid told France's Europe 1 radio.
Belaid's brother Abdelmajid squarely accused Ennahda chief for the murder.
"I accuse Rached Ghannouchi of assassinating my brother," he told AFP.
Last week Belaid accused Ennahda "mercenaries" of attacking his supporters.
On Tuesday he warned of "attempts to dismantle the state and the creation of militias to terrorise citizens and drag the country into a spiral of violence."
His supporters flooded the streets of Tunis and other cities, including Sidi Bouzeid, birthplace of the 2011 revolution.
In central Tunis a national guard tank fired tear gas at protesters, who used bins, coffee tables, barbed wire and barriers to build barricades on Habib Bourguiba Avenue.
Security reinforcements arrived after about 20 minutes of skirmishes, and police wearing helmets and armed with clubs charged some 150 protesters, who fled into neighbouring streets.
In central Tunisia protesters torched the Ennahda party office in Mezzouna near Sidi Bouzeid, ransacked another in the mining community of Gafsa and set fire to a party office in the northeastern town of Kef, witnesses said.
In Kasserine, on the border with Algeria, hundreds of people calling for "vengeance" took to the streets, an AFP journalist said.
In Sidi Bouzeid some 2,000 demonstrated peacefully, but around 200 people tried to storm police headquarters. Police fired tear gas to keep them at bay and the army intervened to try to keep the peace.
Marzouki deplored the killing in an impassioned speech at the European Parliament in Strasbourg that brought tears to the eyes of politicians.
"This odious assassination of a political leader who I knew well and who was my friend... is a threat, it is a letter sent that will not be received," Marzouki said, insisting the murder would not plunge Tunisia into violence.
Belaid's Popular Front along with the opposition Call of Tunisia, Al Massar and Republican Party also called for a general strike on Thursday, when the funeral is expected.
Belaid was a populist known for his iconic smile and black moustache, and who spoke with the working class accent of northwestern Tunisia.
A lawyer, he defended human rights, was jailed under Ben Ali and former president Habib Bourguiba, and was a member of executed Iraqi dictator Saddam Hussein's defence team.
Ghannouchi rejected claims that Ennahda was behind the "cowardly" murder, and told AFP the killing was a "settling of political scores."
The killers "want a bloodbath but they won't succeed," he said.
Prime Minister Hamadi Jebali said a gunman wearing a traditional hooded burnous robe shot Belaid three times at close range as he left his home.
The murder comes at a time when Tunisia is witnessing a rise in violence fed by political and social discontent.