Thousands of protesters gathered in Cairo's Tahrir Square on Friday ahead of a planned rally against the ruling military and presidential candidates who served under ousted leader Hosni Mubarak.

Liberal groups called for the rally before the committee overseeing the election in May last week barred Mubarak's vice president and spy chief Omar Suleiman from standing, along with two leading Islamist candidates.

Islamists, who swept parliamentary elections after Mubarak's overthrow last year, held a rally with similar demands in the iconic square last week avoided by many liberal groups, highlighting rifts since the uprising.

But the powerful Muslim Brotherhood, whose political arm dominates both houses of parliament, said it would throw its weight behind Friday's rally, after its candidate Khairet El-Shater was disqualified.

Supporters of hardline Islamist candidate Hazem Abu Ismail, barred because his mother carried joint US-Egyptian citizenship in violation of the electoral law, also trickled into the square with black flags emblazoned with Islamic slogans.

Suleiman was barred because he did not gather enough endorsements from across the country, as the election law requires.

Abu Ismail's supporters chanted slogans against the military, which took over in February last year, and Mubarak himself, saying the ousted strongman remained in control.

Mubarak is in custody in a hospital, where he is receiving treatment for a heart condition, awaiting the verdict in a corruption and murder trial.

The military has promised to hand over power to a civilian president after the election results are announced in June, but its critics accuse it of angling to stay in power through a proxy leader.

Ahmed Shafiq, a former air force chief who served as Mubarak's prime minister, is still in the race, along with Amr Mussa, a former foreign minister under Mubarak and Arab League chief.

The Brotherhood is fielding the leader of its political wing, the Freedom and Justice Party, after the election committee disqualified Shater because of a military court conviction during the Mubarak era.