Unknown attackers threw petrol bombs at tents housing protesters outside the Egyptian presidential palace in Cairo on Saturday, injuring dozens of people, witnesses said.
A Reuters reporter saw several tents on fire and ambulances driving away from the scene late on Saturday evening. A doctor in one of the ambulances told Reuters that dozens of people had been injured, including police officers.
Protesters opposing an Islamist-backed constitution have been camped outside the residence of President Mohamed Mursi for weeks.
In December, Mursi signed into law the first constitution since the overthrow of veteran autocrat Hosni Mubarak in a 2011 revolution.
The text, which 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 Mubarak, and has prompted often violent protests on the streets of Cairo.
Black plumes of smoke billowed above the area in central Cairo on Saturday night and police reinforcements were deployed outside the palace.