MOSCOW (AP) — A Russian activist was detained Monday after he chained himself to the front doors of the nation's top security agency to protest its handling of a controversial criminal case.

Last week, a Russian court convicted seven members of the left-wing youth group Set (Network) of terrorism and sentenced them to prison terms ranging from six to 18 years.

Human rights activists and Russian opposition members have accused the Federal Security Service (FSB), the main KGB successor agency, of fabricating the case by torturing the suspects into making self-incriminating confessions.

Some of the defendants initially pleaded guilty to some charges but later said they had given false confessions under torture by electric shock.

On Monday, activist Konstantin Fokin chained himself to the entry doors at the FSB's Moscow headquarters to protest against the case. Within minutes, he was detained by police.

Fokin later said on Facebook that police accused him of staging a picket that obstructed passage. He was ordered to stay in custody until Tuesday when a court is expected to hear his case.

Also Monday, a group of Russian artists and curators published a letter accusing the FSB of torturing suspects.