* 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 Tunisair cancelling its flights on Friday in paragraph17)
By Tarek Amara
TUNIS, July 25 (Reuters) - Tunisian opposition politicianMohamed Brahmi was shot dead on Thursday in the second suchassassination this year, setting off violent protests againstthe Islamist-led government in the capital 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 outside their home in Tunis.
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 assassination drew swift international condemnation.
The U.S. State Department called for a "transparent andprofessional" investigation, while French President FrancoisHollande urged Tunisians to show "the necessary spirit ofresponsibility to preserve national unity and guarantee thepursuit of the democratic transition".
U.N. human rights chief Navi Pillay called for the killersto be brought to account and said the government must do more to"deter these terrible acts" and protect people at risk.
Thousands of people protested outside the Interior Ministryin Tunis and a hospital in the Ariana district where Brahmi'sbody had been taken. "Down with the rule of the Islamists," theychanted, 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 some windows of the hospital in Ariana, witnesses said.
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."
Police fired teargas to disperse protesters who stormed alocal government office in the Mediterranean port of Sfax, 270km (170 miles) southeast of Tunis, witnesses said.
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".
National airline Tunisair said it had cancelled all itsflights to and from Tunisia on Friday in response to the strikecall. An airport spokesman said a decision would be taken soonwhether to suspend all flights to the country, which is tryingto renew its popularity with holidaymakers.
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 were 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.
"INTO THE UNKNOWN"
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.
Prime Minister Ali Larayedh condemned the assassination, butsaid: "We are against all the calls to dissolve the governmentand the Constituent Assembly to create a (power) vacuum."
He also drew a link to the upheaval in Egypt, saying in atelevised address that the assassins had aimed to use eventsthere "to try to undermine our process and derail it, and takethe country into the unknown, whether it is chaos, fighting orcivil war or a return to despotism or a return to square one".
Rached Ghannouchi, the Ennahda party leader, said the attackon Brahmi was aimed at "halting Tunisia's democratic process andkilling the only successful model in the region, especiallyafter the violence in Egypt, Syria and Libya".
"Tunisia will not follow the Egyptian scenario," he toldReuters. "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.
The assassination occurred on Tunisia's Republic Day as thecountry prepares to vote in the next few weeks on the newconstitution before a presidential election later in the year.
The turmoil dealt another blow to efforts to resuscitateTunisia's vital tourism industry. Cultural shows, including theCarthage Festival, were suspended following Brahmi's killing.
Secular President Moncef Marzouki appealed for calm anddialogue, but Tunisia looks set for further instability.
Despite recent progress on the constitution, Brahmi'skilling is likely to reactivate the mass protests and strikesacross multiple sectors that followed Belaid's assassination,wrote Anna Boyd, senior Middle East analyst at IHS Country Risk.
Samir Gadio, at Standard Bank, said he expected politicaland economic conditions to deteriorate further in the next fewmonths, saying Ennahda was losing popularity and might seek todelay adoption of the constitution and the holding of elections.
"While Tunisia's army is not influential in the politicalprocess, unlike in Egypt, the risks of an institutionaldislocation or even a military intervention are increasing," hesaid. (Additional reporting by Fatma Matoussi; Writing by AlistairLyon; Editing by Alison Williams)