Thousands of Tunisians gathered in the capital Tunis for the burial of murdered opposition leader Chokri Belaid.
Trade unions have called a general strike in protest at Belaid's assassination, which opposition groups have accused the ruling Ennahda party of helping to plot.
Tens of thousands of people accompanied Belaid's coffin as it was carried to a cemetery near his home, many holding pictures of the dead man and chanting slogans against Tunisia's Islamist-led government.
Soldiers surrounded the procession and military helicopters flew overhead. There were reports of police firing tear gas outside the cemetery, which Tunisian television said was due to gangs of men throwing rocks and attempting to steal cars, according to the Associated Press.
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The rest of Tunis was on lockdown as banks, factories, shops and public transport shut down for the general strike – Tunisia's first in 35 years, according to the BBC. Dozens of flights in and out of the capital airport were also cancelled.
Thousands more people are expected to join protests in the capital and other cities throughout the day.
Hundreds of riot police have been deployed in central Tunis, and there are reports of officers firing tear gas at protesters in the southern town of Gafsa.
BBC correspondent Wyre Davies said it was "difficult to overestimate the tension" on the streets of Tunisian cities. Belaid's murder has proved the flash point for months of disagreement between the government and the secular, liberal groups who accuse it of hijacking the revolution, he said.
Belaid's widow, Besma Khalfaoui, told reporters that despite her grief she was "proud of the Tunisian people," and hopeful that a new revolution was beginning.
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Tunisia remains in political uncertainty, after Ennadha rejected Prime Minister Hamadi Jebali's move to dissolve the government and form a cabinet of technocrats until elections could be held.
At least four opposition groups, including Belaid's Popular Front, have since pulled out of the constituent assembly, the body charged with drawing up a new constitution, the BBC said.
Today’s funeral marks the beginning of what will be a particularly decisive period for Tunisia. Two years of stalled reforms had already stirred up tensions. Now Tunisians are grappling with a heightened climate of political violence that is threatening the lives of some of their most passionate rights activists.
The manner in which the interior ministry responds to today's clashes will probably either temper or deepen the political crisis, at least in the short-term.
Tunisians are fed up with a police force that at times is unwilling to seek out those who have attacked liberal political figures and rallies in recent months, and at other times is also ready to crack down on those same political opponents. The demonstrators largely blame Ennahda, which it says is steering the interior ministry to foster an environment of insecurity.
If the street battles between police and protestors continue, in Tunis and elsewhere, it is likely that calls for the resignation of the Ennahda government will grow louder and make political negotiations all the more urgent. However, if the violence is muted, the Ennahda government may survive the current impasse.