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" in Tunisia.
The cold-blooded killing sparked outrage, with some 2,000 people gathering outside the interior ministry in Tunis, shouting abuse at Ennahda, which they accuse of being behind the assassination of the 48-year-old leftist leader.
Protesters torched the Ennahda party office in Mezzouna, near the central town of Sidi Bouzid, and ransacked another in the mining town of Gafsa, where they tore up Ennahda flags, AFP journalists and witnesses said.
Some 2,000 protesters also took to the streets of Sidi Bouzid itself, birthplace of the 2011 revolution that toppled ex-dictator Zine El Abidine Ben Ali, where they were met by police tear gas, witnesses said.
Marzouki deplored the killing in an impassioned speech before 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," the president said, insisting the murder would not tip Tunisia to unrest.
"We refuse this message and we will continue to unmask the enemies of the revolution," he said, though Tunisia's path was "paved with hurdles," including "orchestrated verbal violence, burnt preachers" and the murder of Belaid.
A presidential aide said Marzouki had scrapped plans to head from Strasbourg to Cairo to join a summit of the Organisation of Islamic Cooperation and instead would fly home immediately to deal with the crisis.
The family of Belaid, who headed the opposition Democratic Patriots party and was a harsh critic of Tunisia's Islamist-led government, was in no doubt as to who was behind the murder.
"My brother was assassinated. I am desperate and depressed," said Abdelmajid Belaid.
"I accuse (Ennahda leader) Rached Ghannouchi of assassinating my brother," he told AFP.
Ghannouchi rejected the accusations and said the killing was linked to the "settling of political scores".
"(The killers) want a bloodbath but they won't succeed" in creating one, Ghannouchi told AFP.
"We can only condemn this cowardly act, which is aimed at (undermining) the revolution and the stability of Tunisia," he added.
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 Wednesday morning.
Belaid's wife gave a similar account of the killing in an interview with the private radio station Mosaique FM.
Jebali told the same radio station everything possible would be done to swiftly arrest the murderer.
"The Tunisian people are not used to such things. This is a serious turn ... our duty to all, as a government, as a people, is to be wise and not fall into the criminal trap which seeks to push the country into chaos."
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.
Several opposition parties and trade unions have accused pro-Islamist groups of orchestrating clashes or attacks against them.
Belaid's party forms part of the Popular Front coalition of leftist parties that has emerged in opposition to the Tunisia government.