Several people have been taken hostage at a pre-trial detention centre in the Russian capital, Moscow.
The inmates say they are holding 15 people, but prison officials say three guards have been seized.
It is believed the abductions at the facility in the city's south-east were carried out by three prisoners accused of serious crimes, including murder.
Police and special forces have sealed off the area. An operation to rescue the hostages is under way.
Officials denied reports that five people were injured during what was described by Russian media as a riot.
Firefighters and ambulances are positioned outside the centre.