Up to 200 prisoners are said to have taken over parts of Bedford Prison amid reports of a “riot”.
There were also reports of loud bangs or explosions coming from inside the prison.
Police and specially trained guards were called to the Category B prison at about 17:00 GMT.
Ambulance and fire services were on stand-by, although no casualties were reported.
In a recent report on HMP Bedford, inmates claimed it was easier to get hold of drugs than clothes or bedding.
“We do know there’s concerted ill-discipline, if not a riot situation going on at Bedford,” said Steve Gillan, general secretary of the Prison Officers Association.
‘Out of cells’
He said some guards at the prison had retreated to a “safe place” and teams of riot-trained officers had been deployed.
“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,” he added.
Mobile phone footage apparently from inside the prison was posted online, revealing chaotic scenes, with scores of prisoners out of their cells and in the prison’s gangways, shouting and bellowing.
One video showed what appeared to be paper and furniture strewn across an atrium floor, although the footage could not immediately be verified.
However, Mr Gillan said: “The POA has been warning about this situation of violence in our prisons – it would appear it’s coming to fruition.”
Image copyright South Beds News Agency The Ministry of Justice said it had no reports of any prison officers being injured in the disturbance.
A spokesman said: “Specialist staff are on site trying to resolve the situation as quickly and safely as possible. This is very much an ongoing incident.”
A Prison Service spokesman said: “We are clear that prisoners who behave in this way will be punished and could spend significantly longer behind bars.”
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.”
Self-harm
Last week Justice Secretary Liz Truss unveiled a White Paper detailing £1.3bn investment in new prisons over the next five years, and plans for 2,100 extra officers, drug tests and more autonomy for governors.
HM Inspectorate of Prisons also discovered incidents of self-harm had almost doubled from 67 to 121.
In a report on Bedford Prison published in September, almost twice the number of prisoners said it was “easy” to access drugs, compared to a previous inspection in February 2014.
The number saying they had developed a drug problem while at the prison increased from 4% to 14%.
Image copyright South Beds News Agency 
Image copyright South Beds News Agency ‘Unacceptable’
The report found that of 72 recommendations made after the prison was last inspected more than two years earlier, only 12 had been achieved and four partially achieved.
Image copyright South Beds News Agency It also said the use of drugs previously known as “legal highs” was having a “serious impact” on safety at the prison.
The inspection found that the physical condition of the prison, which has been on its current site since 1801, was poor, with many inmates living in cramped conditions.
Peter Clarke, chief inspector of prisons, wrote: “Standards in the prison have declined to unacceptable levels.”
Frances Crook, chief executive of the Howard League for Penal Reform, said after the report was published Bedford was “a good example of everything that is wrong with the prison system”.
The prison held just under 500 male prisoners at the time of the inspection.
