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The BBC's Ian Pannell: "It's another tense day, there's already violence, and Molotov cocktails are being thrown"
Early-morning gunfire has rung out around Tahrir Square in the Egyptian capital, Cairo, killing several people.
Sustained bursts of fire lasted for two hours as anti-government demonstrators tried to stay in control of the square.
Egypt's health minister says five people have died in fighting between pro- and anti-government protesters, which began on Wednesday.
The protesters are demanding that President Hosni Mubarak, who has ruled for 30 years, step down immediately.
"Most of the casualties were the result of stone throwing and attacks with metal rods and sticks. At dawn today there were gunshots," Health Minister Ahmed Samih Farid told state television.
"The real casualties taken to hospital were 836, of which 86 are still in hospital and there are five dead."
The US has urged all Americans in Egypt to leave "immediately".
Mr Mubarak has said that he will serve out his current presidential term, which ends in September, but will not run for re-election.
Tahrir Square focusOn Wednesday, groups fought pitched battles in Cairo, in the worst violence in 10 days of protests.
The unrest has left about 300 people dead across the country, according to UN estimates.
Cairo's Tahrir Square has been the main focus of the protests.
A group of anti-Mubarak protesters remain hemmed in there by barricades. They either cannot leave or have decided not to, says the BBC's Jon Leyne in Cairo.
He cites a retired general who has been speaking to tank crews in Tahrir Square as saying the army was losing patience, and if firing continued from pro-government supporters, it was willing to fire on them.
Those attacking them appear to be either police who have taken off their uniforms or plain-clothes "thugs", our correspondent says.
There were petrol bombs being lobbed during the night and now this morning there's been gunfire.
The military leadership seems deeply uncomfortable with what is happening, adds our correspondent; they do not want to turn on protesters but they are not willing to defy the president either.
'Very tense'
Reports early on Thursday suggest that supporters of President Mubarak have been firing on people in the square from the October Bridge, with unknown numbers wounded.
Speaking to the BBC early on Thursday, Mona Seif, a protester, described the atmosphere as "very tense".
"Every couple of minutes we hear a sequence of gunshots, and it's only on one side of the square, which is the one close to Cairo Museum. This is where the clashes have been going on for more than five hours now, completely non-stop," she said.
Wednesday's violence began when thousands of supporters of President Mubarak surged into the square.
One anti-government protester told the BBC that the pro-Mubarak activists had initiated the fighting.
"They started throwing stones at us," the man, named as Zaccaria, said. "Then some of us started throwing stones at them and then we chased them out of the square. They returned once again with the horses and the whips and the thugs."
Opposition supporters say many in the pro-government camp were paid by the authorities to demonstrate, and allowed into the square by the troops surrounding it.
The two sides pelted each other with stones in running battles lasting for hours.
Egyptian troops refused to intervene, but fired into the air to try to disperse people. On Wednesday, they called for demonstrators to return to their homes, a call which was reiterated by Vice-President Omar Suleiman.
Clashes were also reported in Egypt's second city, Alexandria.
The US State Department has urged all Americans in Egypt who wish to leave to go to airports "immediately", adding that delay was "not advisable".
"Additional US government flights after Thursday are unlikely," the State Department added.
US Secretary of State Hillary Clinton told Egyptian Vice President Omar Suleiman in a phone call on Wednesday that the clashes were a "shocking development" after days of peaceful protests.
She also "underscored the important role that the Egyptian Armed Forces have played in exercising restraint in the face of peaceful demonstrations", the state department said in a statement.
'Attitudes hardening'The BBC's Rupert Wingfield-Hayes says he was handcuffed, blindfolded and interrogated by Egyptian secret police, before being released after three hours.
He says that the authorities' attitude appears to be hardening and the ruling elite are fighting back.
The violence drew condemnation from British Prime Minister David Cameron.
"If it turns out that the regime in any way has sponsored or tolerated this violence, that is completely unacceptable," he said after meeting UN Secretary General Ban Ki-moon in London.
Mr Ban said: "Any attack against the peaceful demonstrators is unacceptable and I strongly condemn it."
If Mr Mubarak does not step down, demonstrators have planned to march on the presidential palace on Friday.
Meanwhile, internet services were returning to the country, having been cut off for days by the government.
