Clashes between Islamists and rivals which began after Friday prayers continued well into the night in Egypt’s second biggest city of Alexandria.
Busses were torched and police used tear gas to break up large crowds gathered outside a major mosque as rocks where hurled by both sides.
More than 30 people were injured in the violence which does not bode well for today’s referendum on the country’s contentious draft constitution.
The ballot has been staggered among Egypt’s 51 million eligible voters. Projections after last week’s turn out points to a win for President Mohamed Mursi.
The opposition say the constitution is divisive and accuses Mursi of pushing through a document which leans towards his Moslem Brotherhood allies and ignores the rights of Christians as well as women.