An incident is taking place involving a number of inmates at Maidstone Prison following reports of a riot, the Ministry of Justice has confirmed.
National resources have been deployed to deal with the incident at the Kent jail.
A spokeswoman for the Ministry of Justice said: "I can confirm there is an ongoing incident involving a number of prisoners and national resources have been deployed. It is confined to one wing."
Maidstone, with an inmate population of about 600, is a category C training prison that predominantly houses sex offenders from the Kent and Sussex areas.
Its website says it aims to "create a therapeutic environment that supports, embraces and empowers change with a primary focus on risk reduction and protection".
The prison also takes in a small number of foreign prisoners with more than 18 months to serve and provides forums with the UK Border Agency.
Kent Police confirmed they were aware of the incident, but a spokeswoman said prison services were dealing with the matter.
Prison Officers Association vice chair Ralph Valerio said he was hearing from officers at the scene that up to 180 prisoners are involved.
Mr Valerio said officers on site had the training and skills necessary to bring the mass riot under control.
"The teams that will be going in there have received extensive training to deal with these sorts of incidents," he told Sky News.
Mr Valerio said he hoped negotiators would be able to subdue the situation.
"But this could be replicated across the estate very quickly," he said.
The trade union official said there are no prison officers actually located on the rioting unit, which is separated from the rest of the prison.
A decision would have to be made whether prison guards go into the wing to try to subdue it, he said.
Mr Valerio said that prisoners have begun rioting following changes in the prison regime which meant they have to spend more time in their cells.
He told Sky News that inmates had come to expect a certain regime at the low category prison, but drastic cuts to the prison service meant they were having to spend more time locked in their rooms.
The trade union official said: "The intelligence we are getting is that the demands are largely based on on the 'curtailment of regime' - the curtailment of regime means more time spent behind the door.
" Try to put yourself in the shoes of the offender - you find yourself spending more time locked up with less time to be able to call your family and less time to be able to have social interaction with the staff and with other offenders on that wing then it can have a detrimental effect.
"As a trade union we have been warning against this for some time. The prison system is going through a tremendous amount of change at a tremendous rate of pace and it's a warning that the rates of change is unprecedented.
"The offenders who live at Maidstone come to expect a certain form of regime because they are considered to be of lower risk so they can probably get a better regime than perhaps elsewhere.
"But unfortunately from what we are hearing at Maidstone is that they are chronically short staffed and the result of that shortage of staff is the regime cut."
He said what the staff are able to deliver has been "drastically cut drastically quickly and we have been warning about that for some time".
Mr Valerio praised the professionalism of prison staff and said they would be well trained in how to negotiate with the inmates.
He said: "We have a contingency plan, the prison service is very, very good at dealing with these sorts of situations and the staff involved in that situation are highly professional.
"What we have to do is find the right people for the area and it will be dealt with in the swiftest time possible."