CAIRO, April 20 (Reuters) - Egypt's state security prosecution on Saturday ordered the detention of 7 people for 15 days on charges of belonging to the "Black Bloc," a black-clad youth group opposed to Islamist President Mohamed Mursi.

Hundreds of apparent supporters of the Black Bloc Egypt emerged in January in the forefront of anti-government protests in Cairo, Alexandria and the Suez Canal cities.

Little is known about the group whose first post on its Facebook page was dated Jan. 21 this year, but it has swiftly garnered over 20,000 online followers. Its slogan is "chaos against injustice" and it says it has one enemy only - the Muslim Brotherhood group from which Mursi hails.

The group's tactics appear to be inspired by Black Bloc protesters in Europe who, dressed in black and covering their faces, have formed a hardcore in anti-globalisation protests.

In January, Public Prosecutor Talaat Abdallah ordered police, army officers and the public to arrest anyone suspected of being Black Bloc members and accused them of being an "organised group that participates in terrorist acts." Last week, Abdallah ordered the arrest of 22 suspected group members.

The state security prosecution ordered the 15-day detention of 7 people it said were Black Bloc members on accusations that the group seeks destruction of the country, the state news agency MENA said. It also placed a travel ban on them.

Some opposition members and youth activists denounced the prosecution's arrest and detention orders, saying the government was staging a deliberate crackdown on liberal youth protesters, often randomly and with fabricated charges, while tolerating acts of violence from Islamist protesters during protests.

(Reporting by Shaimaa Fayed and Ahmed Tolba; Editing by Stephen Powell)