Black Bloc has given security forces until Eid El-Fitr – the last day of Ramadan on 8 August – to disperse pro-Morsi sit-ins or it will take matters into its own hands.
Black Bloc is an anarchist-inspired group of masked protesters that often engages in violent street protests. It is not one unified organisation; rather several groups have adopted its name and tactics.
"The blood of our brothers is precious, regardless of whether they are police, army or civilians," it said.
Supporters of former president Mohamed Morsi have been staging sit-ins in Cairo and Giza calling for his reinstatement after he was removed by the army on 3 July amid mass protests against his rule. They have also conducted marches on a daily basis, obstructing traffic on major routes in the capital.
Seven people were killed and 261 injured on 15 July when pro-Morsi protesters clashed with police in Cairo's Ramsis Square and Giza.
On 8 July, 51 people were killed in clashes between pro-Morsi protesters and the army at the Republican Guard headquarters in Cairo.