Many Egyptians fear the military is trying to entrench its power. Egypt has been ruled by a military council since President Hosni Mubarak was ousted in February. Parliamentary elections are due this month.

The protest, and another in Egypt's second city, Alexandria, are demanding the withdrawal of proposals for constitutional change by the military authorities.

The cabinet wants to declare the military the guardian of "constitutional legitimacy". Critics say the wording suggests the armed forces could have the final word on major policies even after a new president is elected.

The document also introduces clauses that would shield the military from civilian oversight.

Witnesses say that Tahrir Square was split between the Brotherhood's Freedom and Justice Party and the more hardline Islamist Salafi rivals, represented by several political parties.

The two set up separate stages and organised their own speeches and chants, only joining forces for Friday prayers.

The Brotherhood has so far avoided confrontation with the ruling military Supreme Council but is now warning that protests will escalate if plans to give permanent political powers to the military are not scrapped.