Visits have been suspended and a search is underway at Stony Mountain Institution just north of Winnipeg.
Thursday evening the warden of the prison ordered a lockdown and a general search of the facility after three inmates from the medium-security unit had been assaulted.
After an evaluation by corrections staff, the injured inmates were taken to a hospital outside the prison for treatment.
Member of the RCMP and Correctional Service of Canada (CSC) are investigating the incident.
Normal operations at the correctional institution will resume after it is considered safe to do so according to a CSC statement.
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