A 21-year-old U.S. man was killed Friday while photographing clashes in Alexandria between supporters and opponents of Egyptian President Mohammed Morsi.

The man was stabbed in the chest, Egypt's official Mena news agency said, though security sources told Efe that the American may have been killed by birdshot.

An Egyptian man killed earlier Friday in Alexandria was a Morsi supporter, the sources said.

Anti-government militants burned the Alexandria offices of the president's Peace and Justice Party, the political wing of the Muslim Brotherhood.

Dozens of people were injured in clashes around the PJP offices, including 11 police.

Medics working at a makeshift hospital set up inside a nearby mosque said they treated 143 people, most of them suffering from birdshot wounds.

The opposing sides pelted each other with stones and traded accusations of snipers posted on rooftops.

Foes of Morsi also set fire to three PJP offices in the Nile delta province of Dakahliya.

Cairo witnessed continuing mass demonstration by both partisans and opponents of the government.

Critics accuse Morsi and his Muslim Brotherhood of doing little to address poverty and the declining economy, of failing to advance the goals of the 2011 revolution that forced out strongman Hosni Mubarak and of seeking to monopolize power.

The opposition is gearing up for nationwide marches this weekend aimed at driving Morsi from office. EFE