* Second assassination of secular politician this year
* Anti-Islamist protests erupt in Tunis, Sidi Bouzid
* Main Islamist Ennahda party condemns killing
* Brahmi's family accuses Ennahda of responsibility (Adds details, quotes, background, call for strike)
By Tarek Amara
TUNIS, July 25 (Reuters) - Tunisian opposition politicianMohamed Brahmi was shot dead outside his home in Tunis onThursday in the second such assassination this year, setting off violent protests against the Islamist-led government in thecapital and elsewhere.
"This criminal gang has killed the free voice of Brahmi,"his widow Mbarka Brahmi told Reuters, without specifying who shethought was behind the shooting.
Brahmi's sister later accused the main Islamist Ennahdaparty of being behind the killing. "Ennahda killed my brother,"Souhiba Brahmi said. Ennahda has condemned the murder.
The politician's wife said Brahmi had left the house afterreceiving a telephone call. She heard shots and found his bodylying on the ground outside as two men fled on a motorcycle.
Brahmi belonged to the secular, Arab nationalist PopularFront party, whose then-leader, Chokri Belaid, was killed in asimilar way on Feb. 6. His death ignited the worst violence inTunisia since President Zine al-Abidine Ben Ali fell in 2011.
Divisions between Islamists and their secular opponents havedeepened since the popular uprising against Ben Ali, whichunleashed unrest across the Arab world, unseating rulers inEgypt, Libya and Yemen, and leading to a civil war in Syria.
Brahmi, 58, was a critic of the Ennahda-led ruling coalitionand a member of the Constituent Assembly that has drafted a newconstitution for the North African nation of 11 million.
The chairman of the Constituent Assembly said Friday wouldbe a day of mourning for Brahmi.
Thousands of people protested outside the Interior Ministryin the capital, Tunis, and a hospital in the Ariana districtwhere Brahmi's body had been taken after the killing.
"Down with the rule of the Islamists," they chanted, and demanded that the government resign.
Big crowds accompanied Brahmi's body when it was taken laterfor autopsy at another Tunis hospital. Despite the presence ofhundreds of soldiers and police, protesters smashed cars andbroke the windows of the hospital in Ariana, witnesses said.
ENNAHDA OFFICES SET ALIGHT
Similar demonstrations erupted in the southern town of SidiBouzid, the cradle of the Tunisian revolution, where protestersset fire to two local Ennahda party offices, witnesses said.
"Thousands have taken to the streets. People have blockedroads and set tyres alight," said Mehdi Horchani, a resident ofSidi Bouzid. "People are very angry."
Tunisia's biggest labour organisation, UGTT, called for ageneral strike on Friday in protest at Brahmi's killing. Itssecretary-general, Hussein Abbasi, earlier predicted that theassassination would lead the country into a "bloodbath".
The government met to discuss the crisis and Prime MinisterAli Larayedh was expected to address the nation later.
Rached Ghannouchi, leader of the Ennahda party, saidBrahmi's assassination was aimed at "halting Tunisia'sdemocratic process and killing the only successful model in theregion, especially after the violence in Egypt, Syria andLibya".
Tunisia's political transition since the revolt that toppledBen Ali has been relatively peaceful, with the moderate IslamistEnnahda party sharing power with smaller secular parties.
But the government has struggled to revive a stutteringeconomy and has come under fire from secularists who accuse itof failing to curb the activities of radical Salafi Islamists.
The government blamed Belaid's assassination, also carriedout by assailants on a motorcycle, on an unidentified group ofSalafi militants, saying six of them are still on the run.
The Egyptian army's overthrow of Islamist President MohamedMursi on July 3 following mass protests against him has furtherenergised the anti-Islamist opposition in Tunisia.
A protest movement known as Tamarod, modelled on theEgyptian group of the same name that orchestrated the anti-Mursidemonstrations, has called for rallies to topple the government.
But Ghannouchi dismissed the plan. "Tunisia will not followthe Egyptian scenario," he told Reuters. "We will hold on."
After the killing of Brahmi, who had applauded the Egyptianarmy's removal of Mursi, the leader of his Popular Front calledfor non-violent action to oust the Tunisian government.
"We call on the Tunisian people to pursue peaceful civildisobedience in all the cities of the republic to bring down thegovernment and the Constituent Assembly and form a nationalsalvation government," Hamma Hammami said.
Ennahda's secretary-general, Hamadi Jebali, who was forcedto resign as prime minister following Belaid's death inFebruary, condemned Brahmi's killing as "the second installmentin a conspiracy against the revolution and the country".
The latest assassination occurred on Tunisia's Republic Dayas the country prepares to vote in the next few weeks on a newconstitution before a presidential election later in the year. (Additional reporting by Fatma Matoussi; Writing by AlistairLyon; Editing by Alison Williams)