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"Prison officers on the landings have withdrawn to a safe place, so prisoners are out in large numbers.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".
Mr 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."
Our thoughts are with everyone at HMP Bedford Prison, our volunteers are on duty this evening if anyone needs support following the events.— Bedford Samaritans (@BedfordSams) November 6, 2016
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.
A Prison Service spokesman said: "Specially trained staff are working with the emergency services to resolve an ongoing incident involving a number of prisoners at HMP Bedford.
"We are absolutely clear that prisoners who behave in this way will be punished and could spend significantly longer behind bars."
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."
HMP Bedford, which has been on its current site since 1801, currently holds around 500 inmates, according to a HM Inspectorate of Prisons report in September.
The watchdog's report found inmates claimed it was easier to get drugs than clothes or bedsheets at a prison where standards have deteriorated to "unacceptable levels".
A survey found the number of prisoners saying it was easy or very easy to get drugs had almost doubled since the last inspection of the jail in February 2014.
The number saying they had developed a drug problem while at the prison increased from 4% to 14%.
The HMP inspection in May also found that the physical condition of the prison was poor, with many inmates living in cramped conditions. The report detailed damaged furniture, graffiti, shortages of clothing and dirty, unscreened showers.
The report also said: "Arrangements for managing violent and bullying behaviour and supporting victims were weak."
Shadow justice secretary Richard Burgon said on Twitter: "More troubling news concerning our prisons. The Justice Secretary needs to do more urgently to tackle crisis."