Pre-election violence has erupted in Cairo, triggering street battles that killed at least 11 people and prompting several presidential candidates to suspend their campaigns.

Egyptian medical and security sources confirmed the death toll in Wednesday's fighting, which began at dawn when assailants raided a site where hundreds of people, protesting against the military-led government, had camped since Saturday.

The mostly Islamist activists fought back against the assailants, whose identities were unknown. Witnesses say both sides attacked each other with firebombs and stones. Gunfire also was heard at the scene.

Egypt's military rulers initially did not intervene in the fighting. They sent in troops and armored vehicles to separate the two sides after several hours.

Prominent presidential candidate Mohamed Mursi of the Muslim Brotherhood said he was suspending his campaign for two days in solidarity with the victims of the violence. An adviser to another candidate Islamist candidate told the Reuters news agency that Abdel Moneim Abol Fotouh also suspended his campaign in response to the unrest.

Egypt's presidential election is due to begin on May 23. It will be the first such vote since a popular uprising ousted longtime autocratic leader Hosni Mubarak in February 2011.

The protesters outside the defense ministry have been demanding an immediate end to military rule in Egypt.

Many of them are Islamists who were angered by the ruling military's decision to bar ultraconservative Islamist cleric Hazem Abu Ismail from standing in the presidential vote. Egypt's election commission disqualified Abu Ismail because his mother had taken joint U.S. citizenship.

Some liberal pro-democracy activists had joined the Islamists at the encampment in calling for Egypt's ruling generals to step down immediately. The ruling military council has promised to hand power to a newly-elected president by July 1st.