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Thousands of people are staging an unprecedented protest in the Tunisian capital, calling on President Zine al-Abidine Ben Ali to quit immediately.

Police have fired tear gas at the crowds protesting outside the interior ministry in Tunis over corruption, unemployment and high food prices.

Doctors say that 13 people were killed in overnight clashes in the capital.

On Thursday night, Mr Ben Ali - who has governed Tunisia since 1987 - announced he would stand down in 2014.

Human rights groups say more than 60 people have died in recent weeks as unrest has swept the country and security forces have cracked down on the protests.

The demonstrators have called on Mr Ben Ali to go straight away, saying Tunisia cannot have true democracy while he remains in charge. Trade unions have called a general strike.

The ����ý's Adam Mynott in Tunis says there is now a public clamour for change that has become a tide which will be very difficult to hold back.

A ����ý Arabic reporter, Mohamed Ballut, says demonstrations are reportedly taking place in other cities, including Sidi Bouzid in the south, where the protests began four weeks ago.

They started after an unemployed graduate set himself on fire when police tried to prevent him from selling vegetables without a permit. He died a few weeks later.

Mr Ben Ali, 74, is only Tunisia's second president since independence from France in 1956. He was last re-elected in 2009 with 89.62% of the vote.

Tourists going home

Meanwhile, UK tour operator Thomas Cook is pulling out all 1,800 of its customers currently on holiday in Tunisia.

Thomas Cook and another holiday company, Thomson First Choice, are cancelling departures to Tunisia scheduled for Sunday 16 January. However, Thomson are not bringing home visitors already in Tunisia.

Tourism is key to Tunisia's economy and an important source of jobs.

The UK, the US and France are among the countries advising against non-essential travel to Tunisia.

"The situation is unpredictable and there is the potential for violence to flare up, raising the risk of getting caught up in demonstrations," the UK Foreign Office said in .

In his speech on Thursday night, Mr Ben Ali said there was "no presidency for life" in Tunisia. He said he did not intend to amend the constitution to remove the upper age limit for presidential candidates, which would have allowed him to stand for a further term in 2014.

The president, who earlier this week had blamed the unrest on "terrorists", also said he felt "very, very deep and massive regret" over the deaths of civilians in the protests.

He said he had ordered troops to stop firing on protesters except in self-defence, and pledged to take action on food prices, which have gone up fourfold in recent weeks.

Afterwards, Mr Ben Ali's supporters took to the streets of Tunis, cheering and sounding car horns.

In a further overture, Foreign Minister Kamel Mourjane said it would be possible to form a government of national unity involving the opposition. Speaking to France's Europe 1 radio on Friday, he also held out the possibility of early parliamentary elections.

But human rights activist Mohamed Abbou said he believed President Ben Ali was "fooling the Tunisians with promises that have no tomorrow".

On Thursday, US Secretary of State Hillary Clinton warned Arab leaders they would face growing unrest unless they enacted real economic and political reform.

Mrs Clinton was speaking in Doha at the end of a four-nation visit to the Gulf.