Egypt's prime minister has apologised for the fighting between pro- and anti-government demonstrators in Cairo's Tahrir Square, which killed five people and wounded several hundred.
Ahmed Shafiq pledged to investigate the violence, calling it a "fatal error".
The protesters are demanding that President Hosni Mubarak, who has ruled for 30 years, step down immediately.
After further pre-dawn violence, the army has been using its vehicles to separate the feuding factions.
One tank turned its turret towards pro-Mubarak demonstrators who were throwing stones at protesters from a road above Tahrir Square on Thursday afternoon, before advancing towards them along with footsoldiers to help clear the overpass.
Egypt's Health Minister Ahmed Samih Farid said earlier that five people had died in the fighting, which began on Wednesday, and 836 were injured - mostly as the result of stone-throwing and attacks with metal rods and sticks. Petrol bombs have also been thrown.
"This is a fatal error," Mr Shafiq told the privately-owned al-Hayat television.
"When investigations reveal who is behind this crime and who allowed it to happen, I promise they will be held accountable and will be punished for what they did."
"There is no excuse whatsoever to attack peaceful protesters, and that is why I am apologising," he said, urging the protesters "to go home to help end this crisis".
Mr Mubarak has said that he will serve out his current presidential term, which ends in September, but will not run for re-election.
Egyptian opposition leader Mohamed ElBaradei and the Muslim Brotherhood have rejected government calls for negotiations, saying Mr Mubarak must leave office first.
Meanwhile, the leaders of France, Germany, Britain, Italy and Spain have issued a joint statement condemning the violence and calling for a political transition that "must start now".
Tahrir Square focus
On Wednesday, opposing 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 over the past 10 days, according to UN estimates.
Cairo's Tahrir Square has been the main focus of the protests, and a group of anti-Mubarak protesters remains hemmed in there by barricades. During a recent exchange of stone-throwing, there was heavy gunfire and the crowd were chanting and banging drums.
Anti-government protesters said earlier they had detained 120 people with ID cards linking them with police or the ruling party, most of whom had been caught attacking demonstrators on Tahrir Square.
The BBC's Jon Leyne in Cairo cites a retired general who has been speaking to tank crews on the square as saying the army is losing patience, and if firing continues from pro-government supporters, it is 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.
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.
The US has urged all Americans in Egypt to leave "immediately".
Earlier, Mona Seif, an anti-government protester in Tahrir Square, 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," she told the BBC.
Wednesday's violence began when thousands of supporters of President Mubarak surged into the square.
"They started throwing stones at us," said an anti-government protester named as Zaccaria. "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.
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.
If Mr Mubarak does not step down, demonstrators have planned to march on the presidential palace on Friday.