Cairo - A shadowy anarchist group in Egypt has caught the attention of the media and authorities since unrest swept the country, but the number of the masked militants and their influence remain a mystery.
Demonstrators, hooded and masked and dressed in black from head to toe, appeared last week in Cairo and other provinces, calling themselves the "Black Bloc" and using the slogan "Chaos against injustice".
Previously unknown, the group has caused a stir with its dramatic flash mob appearances at demonstrations and rallies - where its members appear at marches without prior warning or public announcement.
Presenting themselves as the defenders of protesters opposed to Islamist President Mohamed Morsi's rule, they reportedly model themselves on anarchist groups of the same name in Europe and the United States.
In a video posted on YouTube, the militants say they want to "confront the regime of the fascist tyrants" - a reference to the Muslim Brotherhood from which Morsi hails.
Photographs showed them celebrating around a police armoured vehicle in flames in the middle of Tahrir Square this week, making "V for victory" signs.
The origins and leadership of the group are shrouded in mystery and the extent of its membership is unknown.
But its presence has still unnerved the authorities.
On Thursday, 18 suspected members of the group were detained, accused of "perpetrating acts of sabotage in the country."
"Inquiries have shown that the Black Bloc is an organised group which carries out terrorist actions," said the prosecutor general's spokesperson Hassan Yassine.
The prosecutor urged "citizens to arrest anyone suspected of membership in this group and hand them over" to the army or police, said Yassine.
He also decided to appoint a judge to investigate a complaint lodged by a lawyer who accused the Black Bloc of torching Muslim Brotherhood offices, and of links to opposition and media figures, Yassine added.
"The Black Bloc is an organized group. [Its members] are troublemakers," lawyer Abed al-Wakil, who filed the complaint, told AFP.
Later on Thursday, the authorities detained a member of the militant group suspected of planning to carry out Israeli-directed acts of sabotage.
One person "belonging to the Black Bloc organisation was arrested inside a building overlooking Tahrir Square carrying Israeli plans to target petrol companies and vital installations, maps of these places and instructions on setting fire to some places," the official MENA news agency reported.
Israel firmly rejected the notion of its involvement in any such plot.
Elsewhere, another suspected Black Bloc member was arrested after placing an order with a clothing factory in the city of Mahalla, north of Cairo, for black masks and outfits for the group, the prosecution said.
Dressed in black
On Wednesday, as protesters demonstrated outside the public prosecutor's office against the arrest order of Black Bloc members, an AFP reporter saw plain clothed police detain several young men dressed in black and force them into a car.
An hour later, police arrested another four people masked and dressed in black.
Authorities have not provided much detail about the group whose Facebook page had more than 36 000 "likes", or supporters.
Some doubt the existence of an organised Black Bloc network.
"There is no such thing as the Black Bloc," said Haitham Nur, 25, who was protesting outside the prosecutor's office on Wednesday and describes himself as a "political activist."
The Black Bloc has emerged in the wake of protests that began in Egypt last week with the second anniversary of the revolt of 2011. At least 56 people have been killed and hundreds injured.
The opposition has called new anti-Morsi protests on Friday, but the Black Bloc has not indicated whether it will join them.