At least five people are in custody following a series of early-morning police raids in east-side Toronto, Peel Region and Hamilton.
In all, 12 search warrants were executed, Toronto police spokeswoman Constable Wendy Drummond said.
A number of townhouses in Scarborough, near Morningside and Finch Avenues, were targeted.
The roundup was co-ordinated by Toronto's gun and gang task force and organized-crime officers.
There was no immediate word on charges but more information was expected later in the day.
In Peel Region, the police street-crime gang unit arrested two brothers aged 18 and 22 on multiple gun and drug charges, shortly after midnight, but a spokesman said those arrests are unconnected to the larger investigation.
Meanwhile, the provincial Special Investigations Unit is scrutinizing an incident, also in Scarborough, in which a man was shot in the lower leg by police at a home near Midland and Eglinton Avenues during a search early Thursday morning.
That incident, too, was unrelated to the other raids that took place overnight.
The SIU probes all police-civilian interactions resulting in death, serious injury or allegations of sexual assault.
The Mississauga-based agency said the wounded man is 23 years old and that the incident occurred on Wainfleet Drive at 12:15 a.m.
The man is recovering in Scarborough's Sunnybrook Health Sciences Centre.