The Brotherhood said all those killed were President Morsi supporters, though this could not be independently verified.

Egypt's leading religious authority warned of "civil war" on Friday, June 28 and called for calm after a member of the ruling Muslim Brotherhood was killed ahead of mass rallies aimed at forcing the president to quit, Reuters reported.

"Vigilance is required to ensure we do not slide into civil war," the Al-Azhar clerical institution said in a statement reported by state media. It blamed "criminal gangs" who attacked mosques for street violence. Clashes linked to the political tensions have killed five and wounded scores in recent days.

The Brotherhood said all those killed were President Mohammed Morsi supporters, though this could not be independently verified.

The ancient Cairo academy, which traditionally maintains a distance from the political establishment, also urged opponents of Islamist President to accept his offer of dialogue rather than pressing on with plans for demonstrations.