HMP Bedford riot: Dozens of inmates removed
About 50 or 60 inmates are understood to have been removed from HMP Bedford after a number of prisoners were involved in a riot.
At least 150 inmates are thought to have been involved in Sunday's incident at the Category B facility; the Prison Officers Association has said 230.
Bedfordshire Police said three men had been arrested in connection with a criminal investigation into the unrest.
Two inmates were taken to hospital for treatment and have returned to prison.
The three men, aged 39, 37 and 26, are being questioned on suspicion of committing offences under the Prison Security Act 1992.
An internal investigation is also taking place into what happened.
Police and specially-trained officers were called at about 17:00 GMT following reports of inmates getting out of cells, seizing keys and starting fires.
Officers from Bedfordshire, Cambridgeshire, Hertfordshire and Essex attended the scene, police said.
Sources with knowledge of what happened said prisoners on three wings A, B and C - gallery-style Victorian landings - were involved. One wing sustained most of the damage.
Prisoners also accessed the centre of the prison, which usually contains prison equipment, staff files and other items such as phones.
The Howard League for Penal Reform said there was an ongoing dispute at Bedford, with officers refusing to unlock prisoners when they felt it too risky.
"Of course that means prisoners are spending more and more time banged up in their cells," said Andrew Neilson from the campaign.
"Personally I think that is partially why we've seen this incident happen, because if you are telling people they will spend 23 out of 24 hours of a day locked in basically a toilet, then you're breeding a lot of boredom and frustration."
There were also concerns about staffing levels in the prison service across the country, the campaign said.
Bedford's last inspection showed it was housing 495 prisoners instead of its certified normal capacity of 322.