During protests in Cairo on Monday, masked youths celebrated around a police armored vehicle in flames in the middle of Tahrir Square, waving their hands in V-for-victory signs.
Their emergence has raised concerns even among fellow members of the opposition, who fear the group could spark Islamist retaliation or that it could be infiltrated to taint their movement. Islamist supporters of President Mohammed Morsi call the bloc a militia and have used it to depict the opposition as a violent force wrecking the nation.
Moreover, some Islamists have threatened to form vigilante groups in response, creating the potential for a spiral of violence between rival "militias."
The bloc's appearance comes amid increasing opposition frustration with Morsi, Egypt's first freely elected president, and the Muslim Brotherhood and other Islamists who critics say have imposed a monopoly on power.
The anger has fueled the explosion of violence that at first centered on Friday's second anniversary of the start of the uprising that ousted autocrat Hosni Mubarak. It accelerated with riots in the Suez Canal city of Port Said by youths furious over death sentences issued by a court against local soccer fans over a bloody stadium riot a year ago. Morsi has struggled to regain control, calling a state of emergency in three Suez Canal-area provinces.
The Black Bloc models itself after anarchist groups by the same name in Europe and the United States that have participated in anti-globalization and other protests.