The Public Prosecution in Heliopolis ordered 21 suspects detained Tuesday after clashes between protesters and security forces in front of the Ettehadiya Presidential Palace. The demonstrations had erupted Monday evening to mark the second anniversary of former President Hosni Mubarak's resignation.
The office ordered police to investigate the suspects, who face charges of rioting and assaulting security forces.
Those arrested have denied all charges and said police arrested them randomly.
Investigators have already submitted the remains of Molotov cocktails collected on the palace grounds and sent them for analysis.
Dozens have been arrested across the country following violent protests on Monday as authorities prepare for talks Wednesday aimed at solving political deadlock and widespread unrest.
Meanwhile, the Cairo Cleaning and Beautification Agency began removing graffiti Tuesday that has marred the presidential palace since December's deadly demonstrations. Most of the work was done at Gate 5, where President Mohamed Morsy enters and exits the premises.
Edited translation from Al-Masry Al-Youm