The United States strongly condemned the assassination of Tunisian opposition politician Mohamed Brahmi and called for a thorough investigation to bring those responsible before the courts.

"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," said State Department spokeswoman Marie Harf in Washington.

Brahmi was shot dead outside his home in Tunis, 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."

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Assassinated politician Mohamed Brahmi's daughter Balkis, centre, holds a Tunisian flag as she mourns his death on Wednesday. (Zoubeir Souissi/Reuters)

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.

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.

Canada's Foreign Affairs Minister John Baird also condemned the attack, saying "opposition voices and a plurality of opinions are the bedrock of any democracy.

"We remain committed to supporting Tunisia in its democratic transition and appeal to all parties in Tunisia to refrain from violence and engage in constructive political dialogue," Baird said in a statement.

He expressed sympathies to families, friends and colleagues of the slain politician and urged the Tunisian government to bring the people behind Brahmi's death to justice.

Strike, day of mourning declared

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.

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

Tunisia's largest labour organization, the UGTT, said it would hold a general strike on Friday to protest against the assassination.

Airline Tunisair has cancelled all flights to and from Tunisia on Friday in following with the general strike.

Protesters inspired by Egypt 

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The Egyptian army's overthrow of former President Mohammed Morsi following mass protests against him has energized the anti-Islamist opposition in Tunisia. (Google Maps)

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 Morsi on July 3 following mass protests against him has energized 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".

With files from CBC News