Troops have fired live bullets and tear gas in the centre of the Tunisian capital Tunis, reports say.
At least one person is reported to have been killed and a large area of the city centre has been closed off by police and the military.
It comes after violent scenes in the capital overnight despite a night-time curfew being put in place.
At least 23 people have died since nationwide protests over poverty and corruption began last month.
The BBC's Adam Mynott in Tunis says the unrest has now reached the central streets of the capital, close to shops, banks and the French embassy.
Police have put up road blocks and have fired tear gas to push back crowds gathering to watch the events, our correspondent adds. There have been reports of looting.
Witnesses said at least one person had been shot and another seriously wounded.
It comes after one protester was reportedly shot dead in a western suburb of Tunis, and three others in a southern area of greater Tunis.
However, the International Federation for Human Rights said eight had died.
President Zine al-Abidine Ben Ali has called the unrest a "terrorist act".
Escalating angerThe protests reached the capital for the first time on Wednesday afternoon, as protesters threw stones and police responded with volleys of tear gas.
An indefinite night-time curfew was put in place but overnight, youths fought battles with security forces and set fire to buildings.
More clashes were reported from at least half a dozen towns across the country, including Sfax, Douz and Thala.
Reuters reported up to 10,000 people were on the streets in Sidi Bouzid.
"It is not just about unemployment any more," one protester said. "It's about freedom of expression, freedom of assembly, all the freedoms."
Switzerland's foreign ministry has confirmed that a woman who had dual Swiss-Tunisian nationality was killed in the north of Tunisia.
Swiss Radio said she was hit in the throat by a stray bullet while watching a protest late on Wednesday in the town of Dar Chaabane.
A foreign ministry statement said the Tunisian ambassador had been summoned, and called on the Tunisian government to "respect dialogue, fundamental liberties and human rights", including freedom of assembly.
French diplomats have also reportedly informed the family of a French-Tunisian academic, Hatem Bettahar, that he was one of two protesters killed by the police on Wednesday in the central town of Douz.
Mr Bettahar, a computer science lecturer in France, had been visiting his mother. A Tunisian teacher has said he has posted on the internet a video showing Mr Bettahar lying in a pool of blood after being shot.
The International Federation for Human Rights says 66 people have died since mid-December, almost three times the government's official figure of 23.
UN High Commissioner for Human Rights Navi Pillay said the deaths were "a result of some excessive measures used, such as snipers [and] the indiscriminate killing of peaceful protesters".
"It is imperative that the government launch a transparent, credible and independent investigation into the violence and killings," she said.
Our correspondent says President Ben Ali has tried to respond to the unrest by sacking the interior minister, ordering the release of most of those arrested during the protests, and promising to look into the allegations of corruption.
But many in Tunis view these assurances as empty words, he adds.
"Tunisia needs global reform as well as the formation of a national unity government more than ever," said the secretary-general of the opposition Progressive Democratic Party (PDP), Maya Jribi.
Unions have also called for a strike in Tunis on Friday as another means to put pressure on the government. Union officials told the BBC that a strike had been observed in the central Kasserine region on Wednesday.
Meanwhile, al-Jazeera TV is reporting that the president has sacked two of his top advisers, Abdelaziz Ben Dhia and Abdelouahab Abdallah. And the AFP news agency quoted opposition officials as saying the Army chief of staff, Gen Rachid Ammar, had been replaced for refusing to order soldiers to put down the protesters.
There are also unconfirmed reports that the president's son-in-law, the billionaire businessman and MP Mohamed Sakher El Materi, has fled the country and taken refuge in the Canadian city of Montreal, where he owns a mansion. Protesters have directed much of their anger at the great wealth and lavish lifestyle of the president's extended family.
Anti-government demonstrations have been rare in Tunisia, where there are tight controls aimed at preventing dissent.
Mr Ben Ali is only Tunisia's second president since the country gained independence from France in 1956. He came to power in 1987 and was last re-elected to a five-year term in 2009 with 89.62% of the vote.
source : bbc.co.uk
