Campaigning for Egypt's first post-uprising presidential poll was temporarily on hold after thugs attacked an anti-military protest near the defence ministry in Cairo, leaving 20 people dead, AFP reported.

Four presidential candidates announced they were temporarily suspending their campaigns over the killings.

The Muslim Brotherhood's Mohammed Mursi told reporters he decided to stop campaigning for 48 hours "in solidarity with the protesters." The SCAF, as the ruling authority, was "the first to be responsible," he said.

His main Islamist rival, Abdel Moneim Abul Fotouh, cancelled all activity for the day, his camp told AFP, while leftist candidates Khaled Ali and Hamdeen Sabbahi also announced they were suspending their campaigns.

In a bid to placate the protesters against military rule, the ruling Supreme Council of the Armed Forces (SCAF) said it was ready to hand power to civilian rule on May 24 if a candidate wins the first round of the election. The army had said previously it would hand over at the end of June.

The dawn assault sparked fierce clashes between the unidentified attackers and the protesters, who have been camping out to demand an end to military rule, with both sides hurling petrol bombs and rocks, the official said. The army deployed troops in central Cairo to quell the clashes, a military source told AFP.

A doctor at a field hospital said 20 people were killed and dozens wounded.

Amr Mussa, a former Arab League chief and now a frontrunner in the race, said Wednesday's violence was "proof of the need to put an end to the transitional period according to the timetable, without delay."

The military chief of staff, Sami Enan, said the army may transfer power to an elected president on May 24 if the vote is decided in the first round.

The presidential election is scheduled for May 23 and 24, and a possible run off for June 16 and 17.