The Royal Newfoundland Constabulary was called to Her Majesty's Penitentiary in St. John's late Monday to deal with what sources told CBC was a "small riot" inside.
The RNC said staff at the penitentiary called for help around 11 p.m. to describe what police described as "a disturbance" on one of the units.
In a statement Tuesday, the RNC said "several specialized resources" were called in to bring the situation under control, which didn't happen until about 4 a.m.
Sources told CBC that half a dozen inmates had been drinking jail-house liquor, ripped a door off a shower and tried to use it as a battering ram to smash through a window to get at another inmate.
CBC was also told the inmates had weapons of some kind and that they somehow flooded the unit and smashed windows.
The RNC said the unit where the incident happened was "extensively" damaged.
Police and guards were were able to break up the ruckus.
The RNC said negotiators brought the incident to a peaceful resolution. Sources told CBC that the inmates demanded cigarettes and were given them.
Sources said three people were taken to hospital. It's believed they were all inmates. Police said one person was taken to hospital with a pre-existing medical condition.
Two police cruisers were seen escorting an ambulance to the Health Sciences Centre just before 4 a.m.