Inmates at the Saint John Regional Correctional Centre were under lockdown most of Monday after an incident that a prisoner advocacy group blames on overcrowding.

Saint John Police are investigating a disturbance at the jail Sunday night.

Police said the two-hour melee started when the 12 inmates of unit 1-B, which is maximum security, refused to cooperate with the evening routine. They caused some damage to a dayroom and several sprinkler heads, triggering the fire alarm.

Superintendent Art Crockett said the lockdown was ordered shortly after the incident began around 10:30 p.m.

No one was injured in the disturbance, but emergency crews stood by for about two hours until the situation was under control.

Denise Durette, with the Elizabeth Fry Society, visits the jail often. She said overcrowding has become a problem on the men's side.

She said she's also hearing complaints about a lack of food, since the province switched to a standardized meal system on April 1.

"They're not fed enough. They're telling us that they're hungry and the length of time they go without food is 15 hours in the weekdays and in the weekend, they go 18 hours," said Durette.

The evening meal is served at 4 p.m. and breakfast comes 7 a.m., she said.

The jail superintendent denies there are problems with food or space.

Crockett said the conflict erupted from people not getting along.

"My best sense is that it's personalities, more than anything else," he said.

Crockett said the jail will conduct its own review of what happened. He said they'll also look at whether certain inmates need to be moved, either into another unit or to another jail.