A Day of Rage turned into an evening of bloodshed as more than 100 people were killed across Egypt on Friday.
A correspondent for Al Jazeera described lines of bodies in a makeshift hospital in the nearby Al-Fateh mosque following the attack on marchers in Cairo's Ramses Square.
A witness said men fired from helicopters on the crowd. Hundreds of people were injured.
The Muslim Brotherhood had called for the marches in protest against the killing of hundreds of protesters on Wednesday as police and security forces cleared sit-ins calling for the re-instatement of the deposed president, Mohamed Morsi.