Members say the group was created in response to Dec. 4 clashes, when Brotherhood supporters attacked a protest sit-in outside the presidential palace, touching off hours of street battles that left at least 10 dead and hundreds injured.
Many in the opposition saw that incident as a turning point, a sign that Islamists and the Brotherhood were willing to use violence against Morsi’s critics.
Monday night, a number of protesters praised the masked men in Tahrir Square.
‘‘They aren’t here for sabotage or vandalism, but to protect us from Brotherhood militias,’’ said Ahmed Ali, an engineer.
Ali said police are now ‘‘suppressing the revolution on behalf of the murderer Morsi ... So we need these men to defend the revolution.’’
Hossam al-Hamalawy, a prominent lefist activist, said the Black Bloc youth are ‘‘sincere, they want change and they have seen their friends get killed... (So) they have decided to take the matter into their own hands.’’
But he said it ‘‘could be dangerous for the revolution,’’ warning that ‘‘this could develop to people carrying arms’’ ostensibly in response to the Black Bloc.
‘‘Those who topple the regime are the masses,’’ not underground groups, said al-Hamalawy, of the Socialist Revolutionaries, a key group behind the anti-Mubarak uprising.
Morsi’s office and the Brotherhood have contended for months that the opposition is using the streets to overturn results of elections that Islamists have consistently won.
Now they point to the Black Bloc as proof their opponents are willing to back violence.
On his Facebook page, Morsi’s assistant for foreign affairs Essam el-Haddad accused the Black Bloc of ‘‘systematic violence and organized crimes across the country’’ and accused the opposition of condoning it.
The Brotherhood in a statement denounced ‘‘groups of thugs, militias of black gangs’’ that it accused of attacks on state institution, police and private property. ‘‘The silence of opposition political parties on such crimes ... indicates their support.’’
Morsi’s more hard-line Islamist allies have been more vehement.
The Black Bloc ‘‘must be liquidated completely. These groups must be dealt with with violence and all force,’’ said Mohammed Abu Samra, head of the political party of Islamic Jihad, which once waged a campaign of militant violence in Egypt.
Some ultraconservatives accused Christians of being behind the bloc, in line with their past attempts to fire up their base with warnings that minority Christians are trying to topple Morsi.
Another former jihadi group, the Gamaa Islamiya, threatened on Sunday to create a vigilante group.
Tareq el-Zomr, a leading figure in the group, said that if security forces don’t bring quiet, ‘‘it will be the right of the Egyptian people — and us at the forefront — to set up popular committees’’ to protect property and ‘‘counter aggression.’’
A Facebook page also claimed the formation of a new militia called the ‘‘Muslims Brigade’’ — though it was not possible to confirm that the group exists.
In a video on the page, a group of masked men holding rifles warned of plots by enemies of Islam and a conspiracy by Christians to turn Egypt into a Christian state and accused the main opposition National Salvation Front of helping ‘‘burn Egypt.’’