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 tomorrow 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.