A Superior Court judge certified a class-action lawsuit against the province over the conditions at a London, Ont. detention centre. (Getty Images)

Ontario's police watchdog is investigating the death of a man who was being held at a police station in Thunder Bay.

The Special Investigations Unit said Thunder Bay police arrested the 44-year-old man, who was in violation of a court order, at 4:30 p.m. on Saturday.

The SIU said the man was intoxicated at the time of arrest.

At about 3 a.m., police said they noticed the man in his cell was in some form of medical distress and an ambulance was called.

Emergency medical responders pronounced the man dead.

The SIU has assigned three investigators and two forensic investigators to probe the circumstances of the death.