An inmate was found dead Saturday after a riot and a blaze swept through a Russian prison under a coronavirus lockdown in eastern Siberia, authorities said.

Trouble erupted at penal colony No 15 in Angarsk on Thursday with authorities blaming prisoners, while human rights activists said inmates self-harmed en masse to protest systematic mistreatment.

On Friday, the work yard of the colony was engulfed in flames as riot police cordoned off roads leading to the prison, turning away independent observers. The fire was extinguished by Saturday morning.

"When they cleared the debris, they found the body of an inmate," said Irkutsk region rights ombudsman Viktor Ignatenko, quoted by Interfax news agency. "It was a violent death."

The regional penal service issued a statement late Friday saying the situation is "under control" and investigators are on site after "inmates set several buildings in the work yard on fire."

It confirmed several inmates self-harmed using glass from broken surveillance cameras.

Independent rights monitors called the clashes a "riot" which broke out after a guard beat an inmate.