The presidential campaigns of Khaled Ali and Hamdeen Sabbahi each announced their candidates will be joining a march on Abbasiya Square heading from Al-Fath Mosque at 5pm Wednesday to urge an end to deadly clashes after unidentified assailants attacked Wednesday morning a sit-in in front of the Ministry of Defence.

The campaign of candidate Abdel-Moneim Abul-Fotouh announced its participation in the march and called on people to join it to form a human shield at the sit-in site in the hope of halting clashes.

At least 11 people were killed and dozens injured when plainclothed assailants attacked protesters camping outside the Ministry of Defence early Wednesday morning.

The four-day sit-in outside the ministry was initially prompted by popular anger over Hazem Abu-Ismail's disqualification from the presidential race, but later turned into a protest against military rule.

Assailants attacked protesters in the early hours of Wednesday with homemade bombs, stones, Molotov cocktails, teargas and shotguns loaded with birdshot, according to eyewitness reports.

It was the second major attack on protesters in less than 72 hours. An attack on Saturday reportedly left one protester dead and dozens injured.

The first round of voting in Egypt's presidential elections will take place 23-24 May, according to the schedule announced by the Supreme Presidential Electoral Commission.