Specialist prison officers were dealing with a disturbance on Saturday at a low-security British prison, the government has said.
The Prison Service said an unspecified number of prisoners was involved in an incident of "concerted indiscipline" at Ashwell prison near Oakham, about 100 miles north of London.
"The perimeter is secure and police are in attendance outside the prison," it said in a statement. It gave no further details.
The prison, with a capacity of around 600, is a Category C prison, meaning it is relatively low security.
The Prison Service website says "Ashwell is best suited to those Category C prisoners who are able to display the maturity and self-control to live reasonably in a regime where there is a high expectancy of good behaviour."
Inmates have unsupervised free movement, unsupervised access to the prison grounds and have keys to their own room, it said.