A lockdown was put in place Wednesday at the Saskatchewan Penitentiary following what Corrections Canada was calling "a major disturbance" and what the union representing guards was calling "a riot."

"It was a flat-out riot," said James Bloomfield of the Union of Canadian Correctional Officers. "There are serious injuries and several inmates at outside hospitals right now.

"There's been no staff hurt. Control has been gained at the institution. We're working through how we're going to be operating from here going forward."

The lockdown was first instituted in the medium-security unit at about 1 p.m. Wednesday, and then expanded to the maximum-security unit at 3:30 p.m. as a precautionary measure.

Visits to the prison in Prince Albert, Sask., were suspended.

"It's still locked down," Jeff Campbell, spokesman for the Correctional Service of Canada, said late Wednesday.

"It's been the scene of a major disturbance, so as I say, we're locked down in the interests, of course, of safety and security at the institution for the staff and inmates and the general public as well."

He said to his knowledge, the involved prisoners had been confined to their cells.

"A lockdown takes place when there's a clear and substantial danger to safety and security at an institution," Campbell said. "Normal operations are suspended for the moment but they're going to be resumed as soon as it's considered safe to do that."

He said he did not know whether any injuries had been sustained by either prisoners or guards during the incident.

Campbell said he did not know what had sparked the incident but Bloomfield said tensions at the prison had been building "for a short period of time. It escalated today, severely."

He said the union's critical incident stress management team had been deployed to Prince Albert to help the guards and other staff, and the union's employee assistance program will also be made available to those affected.

The prison has been the scene of a number of escapes in recent years. This year, convicted murder Roger Joseph Gillet got out but was recaptured quickly.

In 2015, there were six escapes, though all were recaptured within a couple of weeks.