Vancouver police say a 51-year-old man was arrested at about 7:30 p.m. PT after causing a disturbance along West 7th Street near Laurel Avenue.
Police say the man was drunk, but couldn't be taken to a nearby detox centre because he was barred from the facility.
Officers took the man home, where they realized he was unconscious.
The man, whose name has not been released, died later in hospital.
Six members of the IIO team are investigating.
This is the fourth incident the IIO has been called in to investigate since it began operating a month ago.