CAIRO — An unpredictable new element has entered Egypt's wave of political unrest: a mysterious group of masked young men called the Black Bloc who present themselves as the defenders of protesters opposed to the Islamist president's rule.

They boast that they're willing to use force to fight back against Islamists who have attacked protesters in the past — or against police who crack down on demonstrations. The youths with faces hidden under black masks have appeared among stone-throwing protesters in clashes with police around Egypt the past several days in the wave of political violence that has shaken the country.

During protests in Cairo on Monday, Jan. 28, 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, Jan. 25, the 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 Canalarea 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 the past decade.

In Egypt, the group's secrecy and self-professed dispersed structure make it difficult to determine its actual scope. It communicates mainly by online social media. Its members' identities are unknown and faces unseen, so it's impossible to confirm the authenticity of those who claim to speak in its name.

It's even impossible to know whether every masked young man in the streets belongs to the block or is just a protester hiding his face — or if the distinction even matters. In Tahrir, on Jan. 28, vendors were selling black masks that young men crowded to buy.

"We are the Black Bloc ... seeking people's liberation, the fall of corruption and the toppling of the tyrant," proclaimed a video announcing the group's formation, posted online Jan. 24. It showed youths dressed in black marching in lines in the Mediterranean city of Alexandria.

"We have arisen to confront the fascist tyrant regime of the Muslim Brotherhood with its military wing," the video said, warning police not to interfere "or else we will respond without hesitation."

Brotherhood officials, Islamist politicians and progovernment media accuse the group of violence ranging from trying to set fire to the presidential palace and attacking Brotherhood offices to ransacking state buildings, blocking train tracks and even exchanging gunfire with riot police.

The mayhem of the past several days has seen such incidents — but it is unclear what role Black Bloc members have had, or whether claims the group is armed are true.

Security officials say they arrested one suspected bloc member carrying ammunition in Cairo on Sunday, Jan 27.

The state-run Al-Ahram newspaper, which has depicted the group as fueling violence, said that Black Bloc members tried to break into a five-star hotel near Tahrir, and fired guns in the air when other protesters tried to stop them.