Hundreds of people gathered at a mosque in Ramses Square on Friday, after the Muslim Brotherhood, of which Mr. Morsi is a member, appealed to its supporters to join a "march of anger".
The demonstrations are taking place under the slogan "the people want to topple the coup" - referring to the military's removal of Mr. Morsi on 3 July.
The protests quickly became violent - the BBC's Jeremy Bowen says the trigger was when a police station came under fire.
He saw at least 12 bodies brought into a mosque near Ramses Square.
Gunfire has also been heard on the banks of the Nile.
Security in the capital is tight, with many armoured personnel carriers on the streets.
The full story, including regular updates, can be found at BBC News.