Riot police fired tear gas at protesters in the Tunisian capital Tunis as thousands massed following the assassination of opposition politician Mohamed Brahmi.

Brahmi, who was a member of parliament, was shot dead in front of his house in the capital on Thursday morning. His death brought thousands onto the streets, with hundreds later clashing with police outside the interior ministry headquarters.

Brahmi, 58, was a member of the People Movement Party, which is part of the same coalition as Chokri Belaid, another prominent politician who was assassinated in February.

"Mohamed Brahmi, general coordinator of the Popular Movement and member of the National Constituent Assembly, was shot dead outside his home in Ariana," state television and the official TAP news agency reported.

"He was riddled with bullets in front of his wife and children," Mohsen Nabti, a fellow member of the small leftist movement, said in a tearful account on Tunisian radio.

Friday will be a day of mourning in Tunisia, the chairman of the Constituent Assembly said. Meanwhile, the country's largest labour organisation announced a general strike on Friday.

All flights to and from Tunisia have been cancelled, the civil aviation authority said.

Navi Pillay, UN human rights chief, condemned the killing of Brahmi and demanded an investigation into his slaying.

"I am shocked and deeply saddened by the news of Mr. Brahmi's assassination. I call upon the authorities to immediately launch a prompt and transparent investigation to ensure that the people who carried out this crime are held accountable," Pillay said in a statement.

The slain politician's widow, Mbarka Brahmi, told Reuters news agency "this criminal gang has killed the free voice of Brahmi," without specifying who she thought was behind the shooting.

Brahmi's sister, Souhiba, meanwhile, accused the main Islamist Ennahda party of being behind the killing.

"Ennahda killed my brother," she said. Ennahda has condemned the murder.

In the southern town of Sidi Bouzid, the cradle of the Tunisian revolution, protesters set fire to two local Ennahda party offices, witnesses told the Reuters news agency.

"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."

'Radical Islamists'

On Wednesday, Noureddin B'Hiri, senior adviser to the prime minister, said that six people believed to have orchestrated the killing of Belaid more than five months ago had been identified.

"We have identified the sponsors and the authors of the assassination of Chokri Belaid," B'Hiri said after a cabinet meeting.

B'Hiri had said that the details would be revealed "soon" by Interior Minister Lotfi Ben Jeddou, without saying when.

Belaid was shot dead outside his home on February 6, in a brazen attack that shocked Tunisians and led to a political crisis that brought down the government of Prime Minister Hamadi Jebali.

The interior ministry blamed the killing of Belaid, who was an outspoken critic of Jebali's ruling Ennahda party, on a cell of "radical Islamists".

In April, the government released the photos and names of five suspects and appealed for help in arresting them.

Since the revolution that toppled the regime of Zine El Abidine Ben Ali in January 2011, "hardline Islamists" have been blamed for numerous acts of violence, notably an attack on the US embassy last September that left four assailants dead and the killing of Belaid.