The coffin is taken in procession towards the cemetery of el-Jellaz in Tunis
Thousands of mourners have gathered in the Tunisian capital, Tunis, ahead of the funeral of assassinated opposition leader Chokri Belaid.
Towns and cities nationwide are braced for further violence and a general strike is being observed.
Mr Belaid, 48, was shot dead at close range on his way to work on Wednesday by a gunman who fled on a motorcycle.
Unions say the government led by the Islamist Ennahda party is to blame for the killing, an accusation it denies.
Prime Minister Hamadi Jebali has tried to defuse tensions by calling for a non-partisan technocratic government. However, his party has refused to accept this.
Some 3,000 people gathered outside the building in the Djebel Jelloud suburb of Tunis where Mr Belaid's flower-covered coffin lay.
Crowds chanted slogans accusing the Ennahda-led government of murdering Mr Belaid.
"With our blood and our souls we will sacrifice ourselves for the martyr," the mourners shouted.
The coffin was then taken on a funeral procession towards the nearby cemetery of el-Jellaz.
Thousands more people are expected to take to the streets of the capital after Friday prayers and ahead of the funeral in the afternoon.
Hundreds of riot police have been deployed in Habib Bourguiba Avenue, the scene of earlier violence.
Elsewhere in Tunis, many shops are shut and most of the public transport is not running.
This is the country's first general strike in 35 years.
A number of flights to and from Tunis-Carthage airport have been cancelled.
Tunisian state TV said universities had been ordered to suspend lectures on Saturday and Sunday, while France said it would close its schools in Tunis.
In Sidi Bouzid, some 10,000 people also gathered to mourn Mr Belaid.
The BBC's Wyre Davies, in Tunis, says it is difficult to overestimate the tension on the streets of Tunis, Sfax and other provincial towns - a tension that has been simmering for many months between liberal, secular Tunisians and the Islamist-led government.
He says that people who thought the violence and division had ended as the Arab Spring swept through the country almost exactly two years ago now find themselves protesting on the same streets, fighting with riot police and accusing the Islamist-led government of stealing their revolution.
The death of Mr Belaid, a leading critic of the governing party, has proved to Tunisians what they already feared, says our correspondent.
Ennahda denies opposition claims that it was behind the killing.
But government critics say that, in recent months, Ennahda has allowed ultra-conservative Muslim groups to impose their will and opinions on what was always regarded as a bastion of Arab secularism.
Mr Belaid was the victim of the first political assassination in Tunisia since the Arab Spring uprising in 2011.
Thousands of people later rallied outside the interior ministry in Tunis, many chanting slogans urging the government to stand down and calling for a new revolution.
In the centre of the capital, a police officer was killed during clashes between police and opposition supporters.
Also on Thursday, in the town of Gafsa, demonstrators observing a symbolic funeral outside the governor's office clashed with police.
One policeman was said to be in a coma on Friday after being dragged from his car and beaten in Gafsa, Agence France-Presse news agency reported.
In Sfax, crowds ransacked a number of shops on Thursday.
Tunisian media reported that more than a dozen Ennahda offices across the country were attacked overnight.
Earlier, four opposition groups - including Mr Belaid's Popular Front - announced that they were pulling out of the country's constituent assembly in protest.
Mr Belaid was a respected human rights lawyer, and a left-wing secular opponent of the government which took power after the overthrow of long-serving ruler Zine el-Abidine Ben Ali.
Current President Moncef Marzouki said the assassination should not affect Tunisia's revolution, cutting short a visit to France and cancelling a trip to Egypt to return home to deal with the crisis.