About 50,000 people are in Tahrir Square, Cairo, to protest against constitutional changes proposed by Egypt's military rulers.
A similiar protest is being held in Alexandria, Egypt's second city.
They are demanding the withdrawal of a constitutional proposal by the cabinet to exempt the army from parliamentary oversight. They also want presidential elections be held no later than April 2012.
Tahrir Square was the focus of 18-days of protests which ended in the toppling of President Hosni Mubarak in February. Egypt has been ruled by a military council since then.
But the BBC reports many people fear the military is trying to entrench its power.
Critics say the wording suggests the armed forces could have the final word on major policies even after a new president is elected.
Parliamentary elections are due to be held on 28 November.