* Chokri Belaid shot in head and chest outside home -party
* PM condemns killing as "assassination of revolution"
* Protests at Interior Ministry, Sidi Bouzid, other cities
* Opposition calls strike, withdraws from constitution body (Updates with opposition reaction, detail on protests, president's comments)
By Tarek Amara
TUNIS, Feb 6 (Reuters) - A fierce critic of the Tunisian government's dealings with radical Islamists was shot dead on Wednesday, sending protesters onto the streets two years after their Jasmine Revolution sparked revolt across the Arab world.
The headquarters of the moderate Islamist Ennahda party, which rules in a fractious coalition with secularists, was set ablaze after Chokri Belaid, an outspoken, secular leader, was gunned down outside his home in the capital.
His party and others in the opposition parties said they would quit the assembly that is writing a new constitution and called a general strike for Thursday when Belaid will be buried.
Prime Minister Hamadi Jebali, who said the identity of the attacker was not known, condemned his killing as a political assassination and a strike against the "Arab Spring" revolution. Ennahda denied any involvement.
As Belaid's body was taken by ambulance through Tunis from the hospital where he died, police fired teargas towards about 20,000 protesters at the Interior Ministry chanting for the fall of the government.
"This is a black day in the history of modern Tunisia ... Today we say to the Islamists, 'get out' ... enough is enough," said Souad, a 40-year-old teacher outside the ministry.
"Tunisia will sink in the blood if you stay in power."
Despite calls for calm from the president, who is not an Islamist, thousands also demonstrated in cities including Mahdia, Sousse, Monastir and Sidi Bouzid, the cradle of the revolution, where police fired teargas and warning shots at protesters who set cars and a police station on fire.
While Belaid's nine-party Popular Front bloc has only three seats in the constituent assembly, the opposition jointly agreed to pull its 90 or so members out of the body, which is acting as parliament and writing the new post-revolution charter. Ennahda and its fellow ruling parties have some 120 seats.
The small North African state was the first Arab country to oust its leader and hold free elections as uprisings spread around the region in 2011, leading to the ousting of the rulers of Egypt, Yemen and Libya and to the civil war in Syria.
But as in Egypt, many who campaigned for freedom from repression under autocratic rulers and better prospects for their future now feel their revolutions have been hijacked by Islamists they accuse of clamping down on personal liberties, with no sign of new jobs or improvements in infrastructure.
Tunisia's new constitution will pave the way for new elections but will inevitably be a source of friction between secularists and Islamists, just as it was in Egypt, where the president adopted sweeping powers to force it through.
The ruling parties have agreed to hold the vote in June, but that date still needs approval by the assembly.
Since the uprising, the government has faced a string of protests over economic hardship and Tunisia's future path, with many complaining hardline Salafists were taking over the revolution in the former French colony once dominated by a secular elite under the autocratic rule of Zine al-Abidine Ben Ali.
Last year, Salafist groups prevented several concerts and plays from taking place in Tunisian cities, saying they violated Islamic principles. That worries the secular-minded among the 11 million Tunisians, who fear freedom of expression is in danger.
Salafists also ransacked the U.S. embassy in Tunis in September, during international protests over an Internet video mocking Islam.
The embassy issued a statement on Wednesday condemning Belaid's killing: "There is no justification for this heinous and cowardly act," it said. "Political violence has no place in the democratic transition in Tunisia."
The United States urged the Tunisian government to bring his killers to book.
Declining trade with the crisis-hit euro zone has left Tunisians struggling to achieve the better living standards many had hoped for following Ben Ali's departure. Any further signs of unrest could scare off tourists vital to an industry only just recovering from the revolution.
"More than 4,000 are protesting now, burning tyres and throwing stones at the police," Mehdi Horchani, a Sidi Bouzid resident, told Reuters. "There is great anger."
Jobless graduate Mohamed Bouazizi set himself on fire in December 2010 in the city, 300 km (180 miles) southwest of Tunis, after police confiscated his unlicensed fruit cart, triggering the "Jasmine Revolution" that forced Ben Ali to flee to Saudi Arabia less than a month later, on Jan. 14, 2011.
President Moncef Marzouki, who last month warned the tension between secularists and Islamists might lead to "civil war", cancelled a visit to Egypt scheduled for Thursday and cut short a trip to France, where he addressed the European Parliament.
"There are political forces inside Tunisia that don't want this transition to succeed," Marzouki told journalists in Strasbourg.
"When one has a revolution, the counter revolution immediately sets in because those who lose power - it's not only Ben Ali and his family - are the hundreds of thousands of people with many interests who see themselves threatened by this revolution," he added.
Belaid, who died in hospital, said earlier this week that dozens of people close to the government had attacked a Popular Front group meeting in Kef, northern Tunisia, on Sunday.
A lawyer and human rights activist, the 48-year-old had been a constant critic of the government, accusing it of being a puppet of the rulers in the small but wealthy Gulf state of Qatar, which Tunisia denies.
"Chokri Belaid was killed today by four bullets to the head and chest," Ziad Lakhader, a Popular Front leader, told Reuters.
The Interior Ministry said he had been gunned down by a man who fled on a motorcycle with an accomplice.
Prime Minister Jebali, a member of Ennahda, said the killers wanted to "silence his voice".
"The murder of Belaid is a political assassination and the assassination of the Tunisian revolution," he said.
Ennahda leader Rached Ghannouchi denied any involvement by his party in the killing. "Is it possible that the ruling party could carry out this assassination when it would disrupt investment and tourism?" Ghannouchi told Reuters.
He blamed those seeking to derail Tunisia's democratic transition: "Tunisia today is in the biggest political stalemate since the revolution. We should be quiet and not fall into a spiral of violence. We need unity more than ever," he said.
He accused secular opponents of stirring up sentiment against his party following Belaid's death. "The result is burning and attacking the headquarters of our party in many areas," he said.
Witnesses said crowds had also attacked Ennahda offices in Sousse, Monastir, Mahdia and Sfax.
French President Francois Hollande said he was concerned by the rise of violence in Paris's former dominion, where the government says al Qaeda-linked militants linked to those in neighbouring countries have been accumulating weapons with the aim of creating an Islamic state.
"This murder deprives Tunisia of one of its most courageous and free voices," Hollande's office said in a statement.
Riccardo Fabiani, Eurasia analyst on Tunisia, described it as a "major failure for Tunisian politics".
"The question is now what is Ennahda going to do and what are its allies going to do?" he said. "They could be forced to withdraw from the government which would lead to a major crisis in the transition."
Marzouki warned last month that the conflict between Islamists and secularists could lead to civil war and called for a national dialogue that included all political groupings.
Ennahda won 42 percent of seats in a parliamentary election in 2011 and formed a government in coalition with two secular parties, the Congress for the Republic, to which President Marzouki belongs, and Ettakatol.
Marzouki's party threatened on Sunday to withdraw from the government unless it dropped two Islamist ministers. (Additional reporting by John Irish in Paris; Writing by Alison Williams; Editing by Alastair Macdonald)