Tunisia is to bury murdered opposition leader Chokri Belaid amid huge tension surrounding his assassination.
Towns nationwide are braced for another day of violence and the largest trade union has called a general strike.
They say the government led by the Islamist Ennahda party is to blame for Wednesday's assassination of Mr Belaid.
PM Hamadi Jebali has attempted to defuse tensions by calling for a non-partisan technocratic government but his party has refused to accept this.
The assassination has exposed many months of tensions between liberal, secular Tunisians and the Islamist-led government, says the BBC's Wyre Davies in Tunis.
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Tunisia in the Arab Spring
- Dec 2010: Trader Mohammad Bouazizi sets himself on fire in Sidi Bouzid, sparking anti-government protests, and later dies
- Jan 2011: Zine al-Abedine Ben Ali stands down as president after escalating protests and clashes
- March 2011: New technocratic government announced by interim president
- Oct 2011: Tunisians vote in first elections of Arab Spring
- Dec 2011: Ex-dissident Moncef Marzouki chosen as president, with Ennahada's Hamadi Jebali as PM
- May 2012: Salafi Islamists clash with police amid a row over alcohol sales
- June 2012: Mr Ben Ali is sentenced to life in prison over the killing of protesters in the 2011 revolution. Saudi Arabia refuses extradition
- Feb 2013: Opposition politician Chokri Belaid shot dead; PM announces plans for technocratic government, a move rejected by Ennahada.
Ennahda denies opposition claims that it was behind the killing.
Our correspondent 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, and Friday's funeral is certain to be an emotional and highly charged event.
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.
Policeman killed
In the town of Gafsa on Thursday, demonstrators observing a symbolic funeral outside the governor's office clashed with police.
Among the protesters were lawyers and judges who have launched a two-day strike in response to the killing.
Earlier, four opposition groups - including Mr Belaid's Popular Front - announced that they were pulling out of the country's constituent assembly in protest.
Tunisian state TV said universities had been ordered to suspend lectures on Saturday and Sunday, while France said it would close its schools in the Tunis.
Protesters have used the killing of Chokri Belaid to call for a new revolution
The first political assassination in Tunisia since the Arab Spring uprising in 2011, Mr Belaid was shot dead at close range on his way to work. The attacker fled on the back of a motorcycle.
Thousands of people later rallied outside the interior ministry, many chanting slogans urging the government to stand down and calling for a new revolution.
In the centre of Tunis, a police officer was killed during clashes between police and opposition supporters protesting against Mr Belaid's death.
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.
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