Stony Mountain Institution was in lockdown on Monday night following an inmate assault earlier in the afternoon.
The lockdown was put in place at about 1:30 p.m., and a search of the institution was being conducted.
Early reports said it was a stabbing involving two people, but Stony Mountain officials and RCMP could not confirm the nature of the injuries or how many people were involved.
RCMP were on the premises and working with Stony's security-intelligence staff to conduct a full investigation, said media spokesman Guy Langlois.
"Criminal and disciplinary charges can be laid against any inmate or individual found to be involved in illegal activity or, in this case, inmate assault," he said.
Ambulances transported several inmates -- the exact number was not disclosed -- to hospital but the injuries were not considered serious.
A press release stated visits were suspended for the medium-security unit until the search was completed.
Stony Mountain Institution is a federal prison for male offenders located about 18 kilometres north of Winnipeg.