Dozens of people have been injured after petrol bombs were thrown at tents of protesters outside the Egyptian presidential palace.
A doctor in one of the ambulances said that dozens of people had been injured, including police officers.
Protesters opposing an Islamist-backed constitution have been camped outside the residence of President Mohammed Mursi for weeks.
In December, Mr Mursi signed into law the first constitution since the overthrow of veteran autocrat Hosni Mubarak in a 2011 revolution.
The text, which Mr Mursi fast-tracked through parliament and a national referendum, was fiercely opposed by many of the liberals, secularists and leftists who had helped to topple Mr Mubarak.
This has prompted often violent protests on the streets of Cairo.
Black plumes of smoke billowed above the area in central Cairo tonight.
Police reinforcements have been deployed outside the palace.
