Quebec's police watchdog was called in to investigate, along with provincial police, after a 51-year-old man was found unconscious in a Montreal police cell in Saint-Laurent.
The watchdog, also known as the BEI, said another detainee was in the same cell and that the man in question was incarcerated late Thursday morning not long after he was arrested.
The BEI probes incidents involving police and a civilian in which the civilian is seriously hurt or even killed or is hurt when police use a firearm.
The man was said to be in critical condition.