A US citizen has been killed in the Egyptian city of Alexandria during clashes between supporters and opponents of President Mohammed Mursi.

A senior security official said the man died from a stab wound to the chest.

The victim was killed in violence near the ruling Muslim Brotherhood's local offices.

A US embassy official said: "The US embassy has heard of the reports of the death of an American citizen and is seeking to confirm them."

State news reports said 70 people had been injured.

The clashes came two days before President Mursi's critics hope millions take to the streets to demand new elections.

Egypt's leading religious authority warned of "civil war" and called for calm ahead of major rallies the opposition hopes can force the Islamist president to quit.

Influential clerics in the Al-Azhar institute - which broadly supports the president -  blamed "criminal gangs" who besieged mosques for street violence which the Muslim Brotherhood said has killed five of its supporters in a week.

The movement's political wing warned of "dire consequences that will pull the country into a violent spiral of anarchy".

It claimed that liberal leaders, including former top UN diplomat Mohammed El Baradei, were personally responsible for inciting violence by hired "thugs" once employed by the ousted dictator, Hosni Mubarak.

Opposition leaders also condemned the violence.

There was no immediate sign of trouble as thousands of Islamists gathered round a Cairo mosque after Friday weekly prayers to show support for Mr Mursi.

His opponents hope millions will turn out tomorrow to demand new elections, a year to the day since he was sworn in as Egypt's first freely chosen leader.