An Egyptian security official says the death toll from clashes between Islamist protesters and security forces the day before has risen to 17.

The official reported the new toll on Saturday. He spoke on condition of anonymity because he was not authorized to talk to media.

Friday's protests gripped Cairo and other heavily populated cities and provinces such as Alexandria, Ismailia, Fayoum and Minya.

They were the most violent in months, coming less than two weeks ahead of a key referendum on an amended constitution.

The Muslim Brotherhood, Egypt's largest Islamist group, and its supporters have called for a boycott of the vote. They have staged near-daily protests since the military ousted Islamist President Mohammed Morsi in a popularly-backed coup in July.