Dmitry Vorobyovsky, a prominent anti-Kremlin protester in the southwestern city of Voronezh, was forcibly detained by paramedics on Friday.

“At the hospital he was tied to a bed for three hours during which he was injected with an unknown substance and given pills against his will,” Amnesty said in a statement Monday.

The rights group said the alleged medical grounds for the 52-year-old’s “detention and forcible treatment” remain unclear and that he “may have been detained because of his outspoken criticism of government policy”.

The local prosecutor said he had asked a Russian committee of enquiry for information on the case.

Psychiatric treatment was widely used during the Soviet period to try to crush dissidents who criticised the system.

Russian authorities have passed a raft of legislation in recent years to make it increasingly difficult to protest against the rule of strongman leader Vladimir Putin.