Mohamed Brahmi, a member of Tunisia's political opposition, was shot dead in Tunis on Thursday.
Brahmi was a member of the secular coalition called the Popular Front, and held a post in an assembly charged with writing the country's constitution.
French newspaper Le Figaro said that the 58-year-old politician was killed by two men on a motorbike who shot him 11 times as he exited his car.
Party officials said Brahmi was shot in front of his wife and children.
He is the second secular politician to be assassinated in Tunisia in the last six months.
Chokri Belaid, head of the secular Party of Democratic Patriots and a strong opponent of the Islamists, was shot dead in February outside his home.
Belaid's death led to a political crisis that rocked Tunisia, which was already in a difficult transition period following the Arab Spring. His murder was pinned on Islamic extremists.
The protests that followed Belaid's death led Prime Minister Hamadi Jebali to resign.
The Tunisian government said that it had identified six people linked to the attack on Belaid.
Meanwhile, the chairman of the Constituent Assembly declared Friday a day of mourning over Brahmi's death.
Tunisian police used teargas to disperse hundreds of protesters who stormed a government office in Sfax, according to witnesses.
Tunisair late Thursday abruptly cancelled all Friday flights in response to a labor strike being organized by the General Union of Tunisian Labor, said Reuters.
Tunisair’s communications director Solaf al-Mokaddem confirmed the move to Saudi Arabia's Al Arabiya, adding that extra flights will be scheduled for Saturday.
The US State Department strongly condemned Brahmi's assassination and called for a transparent and professional investigation.
"This is not the first political assassination since Tunisia's revolution and there is no justification for such outrageous and cowardly acts in a democratic Tunisia," State Department spokeswoman Marie Harf said Thursday.
Radical Islamic groups have been blamed for several attacks around the country, including one on the American embassy in September.