Police and specialist guards have been called to a prison following reports of a riot involving up to 200 inmates.
Emergency services were called to Category B HMP Bedford in the town centre at around 5pm.
The Prison Officers Association (POA) said some guards at the prison had retreated to "safe place" and national teams of riot-trained officers had been deployed.
Steve Gillan, POA general secretary, said: "We do know there's concerted ill-discipline if not a riot situation going on in Bedford.
"There could be anything up to 200 prisoners involved. We don't know how many are out of the cells, some are still locked up.
"Prison officers on the landings have withdrawn to a safe place, so prisoners are out in large numbers.
"We don't know if it will turn into a full scale riot as these things can turn very quickly."
It comes after the head of the POA Mike Rolfe last week warned British jails have been engulfed by a "bloodbath".
Gillan said: "The POA has been warning about this situation of violence in our prisons - it would appear it's coming to fruition.
"I just hope there's no prisoners or indeed prison officers injured in the violence."
A police spokeswoman said: "Bedfordshire Police is currently assisting HMP Bedford following reports of unrest within the prison.
"We were called shortly before 5pm and have a number of officers at the scene working with prison staff to resolve the issue."
It was reported that prisoners started a riot and have taken over parts of the category B prison.
Ambulance and fire services are also on standby outside the prison.
A spokesman for the East of England Ambulance Service said: "We are at the scene on standby at this stage. It's an on-going incident. We are not in the prison."