Prominent Tunisian opposition leader Chokri Belaid was shot dead outside his home in Tunis on Wednesday, sparking angry protests by his supporters and attacks on offices of the ruling Islamist Ennahda party.
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 cold-blooded killing sparked outrage, with thousands of people massing outside the interior ministry in Tunis where they clashed with police and shouted abuse at Ennahda, which Belaid's family accuse of being behind the assassination.
The wife of the 48-year-old leftist leader told a radio station her husband had received death threats and was shot dead before her eyes as he left their Tunis home.
"I saw his blood flowing, I saw his little smile. I saw that they want to kill democracy," Basma Belaid, her voice trembling with emotion, told France's Europe 1 radio.
"For a long time we have been receiving threats, every day without exception. We always tried to be sure that, when he went anywhere, there was always at least one person with him."
His brother Abdelmajid Belaid blamed Ennahda.
"My brother was assassinated. I am desperate and depressed," he told AFP.
"I accuse Rached Ghannouchi of assassinating my brother," he told AFP.
The murder brought thousands of people out on the streets of Tunis and in other cities, including central Sidi Bouzeid -- birthplace of the 2011 revolution that toppled ex-dictator Zine El Abidine Ben Ali.
Police fired tear gas at protesters after they were attacked with bottles and stones outside the interior ministry and used clubs to disperse the angry crowd, AFP journalists said, as clouds of smoke filled the area.
The tension rose further when an ambulance carrying Belaid's body drove through central Tunis, with protesters crowding behind the vehicle and clashes broke out between police and dozens of protesters, the journalists said.
Protesters pressed against the ambulance to protect it from the violence.
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.
They broke furniture and tore up Ennahda flags and set fire to computers to vent their fury.
In Kasserine, on the border with Algeria, hundreds of people calling for "vengeance" took to the streets as schools suspended classes, an AFP journalist said.
In Sidi Bouzeid some 2,000 demonstrated peacefully but around 200 people tried to storm police headquarters and police used tear gas to keep them at bay.
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 tip Tunisia to unrest.
An aide said Marzouki scrapped plans to go to Cairo to join the Organisation of Islamic Cooperation summit and instead would fly home from Strasbourg immediately to deal with the crisis.
Belaid, a popular figure known for his iconic smile and black moustache, was a human rights lawyer who served jail time under ousted dictator Ben Ali, and former president Habib Bourguiba. He was part of ousted Iraqi dictator Saddam Hussein's defence team.
He headed the opposition Democratic Patriots party and was a harsh critic of Tunisia's Islamist-led government.
Ennahda chief Ghannouchi rejected claims that his party was behind the "cowardly" murder, and said the killing was a "settling of political scores".
"(The killers) want a bloodbath but they won't succeed" in creating one, Ghannouchi told AFP.
Prime Minister Hamadi Jebali described the murder as "an act of terrorism".
He said a gunman wearing the traditional hooded long burnous robe shot Belaid with three bullets fired at close range as he left his Tunis home in the morning.
France's President Francois Hollande said the murder had robbed Tunisia of "one of its most courageous and free voices."
The murder of Belaid comes at a time when Tunisia is witnessing a rise in violence fed by political and social discontent two years after the mass uprising that toppled Ben Ali.