Tunisian opposition politician Mohamed Brahmi was shot dead outside his home in Tunis on Thursday in the second such assassination this year, setting off mass protests against the Islamist-led government in the capital and elsewhere.

"He was shot in front of his house when he was with his disabled daughter," Mohamed Nabki, a member of Brahmi's secular, nationalist Popular Party, told Reuters. "The killers fled on a motorbike."

The assassination of another secular politician, Chokri Belaid, on Feb. 6 ignited the worst violence in Tunisia since the 2011 fall of autocratic President Zine al-Abidine Ben Ali.

"This criminal gang has killed the free voice of Brahmi," his widow Mbarka Brahmi said, without specifying who she thought was behind the shooting.

Brahmi was a vocal critic of the ruling coalition led by the Islamist Ennahda party and a member of the Constituent Assembly charged with drafting a new constitution for the North African nation, which is split between Islamists and their opponents.

The chairman of the Constituent Assembly declared that Friday would be a day of mourning for Brahmi.

Thousands of people protested outside the Interior Ministry in the capital, Tunis, after the killing.

"Down with the rule of the Islamists," they chanted, and demand the government resign.

Similar demonstrations erupted in the southern town of Sidi Bouzid, the cradle of the Tunisian revolution, where protesters set fire to two local Ennahda party offices, witnesses said.

"Thousands have taken to the streets. People have blocked roads and set tyres alight," said Mehdi Horchani, a resident of Sidi Bouzid. "People are very angry."

Rached Ghannouchi, leader of the Ennahda party, said Brahmi's assassination aimed at "halting Tunisia's democratic process and killing the only successful model in the region, especially after the violence in Egypt, Syria and Libya".

Tunisia's political transition since the revolt that toppled Ben Ali has been relatively peaceful, with the moderate Islamist Ennahda party sharing power with smaller secular parties.

But the Egyptian army's overthrow of Islamist President Mohamed Mursi on July 3 following mass protests against him has energised the anti-Islamist opposition in Tunisia.

Ennahda's secretary-general, Hamadi Jebali, who had to resign as prime minister following Belaid's death in February, condemned Brahmi's killing as "the second installment in a conspiracy against the revolution and the country".

The secretary-general of Tunisia's main trade union confederation, Hussein Abbasi, predicted a "bloodbath".