Prison officers are said to have lost control of two wings at a jail in Hertfordshire, with staff shortages being blamed for the trouble.
The Ministry of Justice (MoJ) confirmed an incident is ongoing at The Mount prison, and said specially trained staff are dealing with the matter.
The facility has been running on a "restricted regime" in recent weeks and is 47 staff short, prison affairs academic and blogger Alex Cavendish said.
He said two wings, housing a total of 227 prisoners, are now out of control.
Mr Cavendish said he had been told the regime has seen inmates locked up for 24 hours a day in some cases, and food delivered cold to cell doors.
He added that he had warned just days ago that the prison was "on the brink of a meltdown".
He said: "The technical term is that the staff have lost control of two wings. What 'lost control' means is that the prisoners are basically rioting, in layman's terms."
He said so-called Tornado squads, equipped to deal with riots, had been sent in.
The Category C male prison, which opened in Hemel Hempstead in 1987, has a population of more than 1,000 prisoners.
The facility is described as a "hybrid training and resettlement prison" which caters for a number of prisoners nearing the end of their sentences who are residents of the county and nearby areas.
An inspection in 2015 found the prison was performing well, with inspectors noting it was "reasonably safe and felt calm and well ordered", although then chief inspector of prisons Nick Hardwick added that there was "room for improvement".
A spokeswoman for the MoJ said: "Specially trained staff are working to resolve an incident involving a number of prisoners at HMP The Mount.
"The prison is completely secure and there is no risk to the public."