A riot involving "around 40" inmates at Maidstone Prison has been resolved.
The incident on Saturday evening was confined to one wing of the prison, and was resolved with no injuries to staff or prisoners.
But the Prison Officers Association vice chair Ralph Valerio told ITV News that officers "feared for their lives" and were forced to retreat.
Mr Valerio said inmates had been "smashing up" parts of the Thanet Wing and described Saturday's event as a "riot".
"An investigation is under way and the perpetrators will be dealt with appropriately by the prison," the Prison Service said.
Maidstone is a "category C" prison, meaning it is for inmates considered unlikely to attempt to escape. It is the third out of four security categories, ranging from A for the most dangerous to D for the least dangerous prisoners.
The prison houses sex offenders with over 12 months left to serve, and foreign nationals with a minimum of 18 months left to serve.
According to Mr Valerio, the current staff to inmate ratio at HMP Maidstone is 1:30.
He attributed the unrest to cuts of £90 million to prison services in recent years.
"The reduction in staff numbers means there are fewer prison officers available to carry out a normal prison regime. This means that prisoners spend more time locked up in a contained space.
The Prison Service also reported a disturbance at Rye Hill Prison, near Rugby in Warwickshire.
"There was a passive demonstration at HMP Rye Hill where around 60 offenders refused to return to their cells," a spokesperson said.
The organisation said the disturbance was resolved peacefully, and that it was being considered as unrelated to the Maidstone Prison incident.