A prisoner took to the roof of HMP Northumberland on Saturday, in what one witness described as a "protest".
A spokesperson for the prison confirmed the incident, which was still going on at 6pm on Saturday evening.
The man had taken up a position on the roof of the Acklington side of the jail.
One prisoner told Chronicle Live: "One of the prisoners has got onto the roof of the jail protesting and is surrounded by security staff on the ground."
A spokesperson for HMP Northumberland, which is run by Sodexo, said: "A prisoner is on the roof of one of the prison buildings and prison staff are trying to persuade him to come down safely."
The Ministry of Justice said the incident at the privately-run jail was being managed by Sodexo.
In the past, HMP Northumberland has been put in the spotlight over drug use and violence among inmates.
However, a recent inspection by the prison's Independent Monitoring Board found that efforts to reduce violent incidents had proven effective.
Body-worn cameras, 'violence reduction mentors' and continued emphasis on making sure any misbehaviour was followed up on and met with consequences had helped with this, the report said.
