A fatal blast during a protest against Egyptian President Mohammed Mursi in the Suez Canal city of Port Said was caused by a home-made hand grenade, security sources said today.

After the blast late last night, which killed one man, traces of an explosive substance were found on the bodies of some of the 15 wounded. Ballistics tests are still under way.

Hundreds of anti-government protesters had gathered in a central square in the city, at the Mediterranean end of the canal, one of several small demonstrations around the country ahead of mass rallies tomorrow aimed at unseating Mr Mursi.

Elsewhere, two people, one an American student, were killed after protesters stormed an office of Egypt's ruling Muslim Brotherhood in Alexandria.

Egypt's leading religious authority this week warned of "civil war" after violence in the past week that had already left several dead and hundreds injured.

It backed President Mursi's offer to talk to opposition groups ahead of tomorrow's protests.

The United Nations, European Union and United States have appealed for restraint and urged Egypt's deadlocked political leaders to step back from a confrontation threatening the new democracy that emerged from the Arab Spring revolution of 2011.

The US embassy said in a statement it was evacuating non-essential staff and family members and renewed a warning to Americans not to travel to Egypt unless they had to.

The Muslim Brotherhood said eight of its offices had been attacked yesterday, including the one in Alexandria.

Officials said more than 70 people had been injured in the clashes in the city.

One was shot dead and a young American man who was using a small camera died after being stabbed in the chest.

He was identified as Andrew Pochter, a 21-year-old student from Chevy Chase, Maryland who had been studying at Ohio's Kenyon College.

The college said he had been working as an intern for the U.S. educational organisation AMIDEAST.

A Brotherhood member was also killed overnight in an attack on a party office at Zagazig, in the heavily populated Nile Delta, where much of the recent violence has been concentrated.