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 wrestlers' masks have appeared among stone-throwing protesters in clashes with police around Egypt the past five days in the wave of political violence that has shaken the country.

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 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 Monday, 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 Thursday. 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 pro-government 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 five 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.

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.

A university graduate named Sherif el-Sherafi said he was a founder of the group in an interview with the El-Watan newspaper - though his claims could not be independently confirmed.

He said the bloc has 10,000 members nationwide, organized into groups of around 20 each, but with no chain of command. Members are trained in self-defense and how to deal with tear gas.

"Violence is not an action but a reaction," he said. He depicted the situation as an inevitable clash between the opposition and government. "What is coming is worse."

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."