The Egyptian army on Monday released footage of what it said were Islamist militants mixing with demonstrators and shooting the armed forces.
The Muslim Brotherhood has accused to the army of shooting at peaceful protesters and killing dozens in front of the Republican Guard headquarters.
The military said it was responding to gunfire from militants who infiltrated demonstrator.
Footage released by the army shows a number of masked and armed men attacking army and police personnel.
The army and the Muslim Brotherhood traded blames over responsibility for the killing, which left at least 51 people dead.