Stony Mountain Institution north of Winnipeg is in lockdown after three inmates were assaulted on Thursday night.
The inmates were in the medium security unit at around 7:40 p.m. when the assault occurred. They were transferred to an outside hospital for treatment.
Stonewall RCMP and the institution are investigating.
No staff members or other inmates were injured during this incident.
A search of the medium and maximum security units is underway and visits have been suspended until operations are considered safe to resume.