Inmates are reported to have taken over part of a prison wing.
Officials at HMP Northumberland confirmed a "disturbance" was under way, but dismissed as "speculation" the suggestion of a stand-off between inmates and guards.
A spokesman for Sodexo, which operates the prison, said: "We can confirm there is a disturbance at HMP Northumberland.
"It is confined to part of one wing of the prison and we are working to resolve the situation as quickly as possible."
The incident has been ongoing since around 7.30pm.
The Sodexo spokesman added: "It is not quite resolved yet."