The BBC's Helena Merriman says police were beating protesters - "about 20 or 30 of them I saw, covered in blood"
Thousands of Egyptian protesters remain in Cairo's Tahrir Square after two days of clashes in which at least 13 people were killed and hundreds injured.
On Sunday, police and troops made a violent attempt to evict the demonstrators, firing tear gas and beating them with truncheons.
However, the protesters returned less than an hour later, chanting slogans against Egypt's military rulers.
The European Union said it condemned the violence "in the strongest terms".
There were also clashes in other cities including Alexandria, Suez and Aswan.
A total of 11 people were reportedly killed on Sunday and two on Saturday, according to medical sources. Health officials say as many as 900 have been injured, including at least 40 security personnel.
The demonstrators, some wearing gas masks, say they fear Egypt's interim military rulers are trying to retain their grip on power.
The violence comes a week before the country's first parliamentary elections since President Hosni Mubarak was overthrown in February.
Armoured vehicles
The European Union's foreign policy chief Catherine Ashton called on the Egyptian authorities to cease violence against the protesters.
"I urge calm and restraint and condemn the use of violence in the strongest terms. There is no doubt that the transitional process is a difficult and challenging one," she said.
"I reiterate that the interim authorities and all parties concerned have the crucial task of listening to the people and protecting their democratic aspirations."
A second day of violence began on Sunday when stone-throwing protesters advanced from the square - focal point of February's uprising - towards the interior ministry.
Officers fired volleys of tear gas and drove the protesters back, before blocking the street leading to the ministry.
Armoured personnel carriers brought in reinforcements as the security forces tried to gain the upper hand.
Scores of soldiers and police poured into the square, beating protesters and dismantling a protest camp there.
But within an hour, protesters swarmed back into the square, usually one of Cairo's busiest traffic thoroughfares.
The BBC's Helena Merriman at the scene says the atmosphere is tense, with moments of calm punctuated by outbreaks of panic and running.
The edges of the square are thinning out but the road to the ministry of interior is full of protesters, she says.
In recent weeks protesters - mostly Islamists and young activists - have been holding demonstrations against a draft constitution that they say would allow the military to retain too much power after a new civilian government is elected.
They have repeatedly tried to gain a foothold in Tahrir Square again, but until this weekend they had always been removed quickly by the police.
"The violence [on Saturday] showed us that Mubarak is still in power," one protester, Ahmed Hani, told the Associated Press news agency.
Protester Magdy Mohamed Ali: "If they think that the army personnel can bring us down then they are in for surprise"
He said the leader of Egypt's military government, Field Marshal Hussein Tantawi, should resign.
"We have a single demand: the marshal must step down and be replaced by a civilian council," he said.
The latest violence is some of the worst in months between the Egyptian authorities and demonstrators.
Parliamentary elections are due to begin on 28 November and take three months.
Earlier in November, Egypt's military rulers produced a draft document setting out principles for a new constitution.
Under those guidelines, the military would be exempted from civilian oversight, as would its budget.
This has angered protesters who fear the gains they have made during the uprising could yet slip away as the military tries to retain some grip on power.
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