More than 200 prisoners have been shipped out of HMP Bedford - including 50 riot "ringleaders" - as two wings are said to be out of action.
But the Prison Officers Association (POA) today insist inmates had not been violent towards staff and were simply "letting off steam" about poor conditions.
Michael Rolfe, national chair of the association, said the jail has a total population of about 500 inmates. So far 220 of them have been sent to sites around the country.
Fifty "ringleaders", thought to be responsible for the six-hour rampage, were moved out on Sunday night while the rest are being transferred today.
A number of hygiene vans have been seen going into the prison and the POA said two wings were thought to be out of action.
Mr Rolfe said the trouble began after officers began trying to get inmates back into their cells after their scheduled time out.
Officers had warned of plans by prisoners to cause trouble and had urged management not to let them out, but had been overruled.
But Mr Rolfe said officers had not been subjected to any violence or threats - and that the protest was largely aimed at "making a lot of noise about the state of the prison" and "the squalid conditions they are being kept in".
Once specialist riot officers were brought in, the prisoners quickly gave up and returned to their cells.
The Prison Officers Association warned this latest disturbance would make it even more difficult to recruit - as the government pledged just last week to increase staff numbers.