The Prison Service said a “disturbance” started on the site at 1930 GMT Inmates are involved in a third night of rioting at a prison complex in South Yorkshire.
A prison source told the BBC prisoners were rioting in the adult section at Moorland prison, near Doncaster.
Fire engines, police and ambulances were at the scene and extra prison officers were seen entering the jail.
Violence flared at the adjacent Moorland Young Offenders Institution after inmates refused to return to their cells on Tuesday and Wednesday.
As a result of damage caused to the buildings, 86 inmates were earlier transferred to other prisons.
A Prison Service spokesman confirmed a “disturbance” started on the adult side of the site at 1930 GMT.
He said: “Trained prison service staff are dealing with the disturbance and Tornado teams (prison officers in riot gear) are on their way to the establishment.
“This is a very serious matter and a high level review will be started tomorrow.”
More than 80 inmates from the young offenders institution have been transferred to other prisons A spokesman for the Prison Officers’ Association (POA) said: “The incident is taking place at this very moment and we don’t know what we’re dealing with. No two incidents are the same.
“It is being dealt with by prison officers and it’s all hands to the pump tonight.”
Wednesday night’s incident at the young offenders’ unit started when about 55 prisoners refused to return to their cells at 1830 GMT.
They surrendered more than eight hours later after the Tornado teams were called in.
One inmate was taken to hospital with head injuries but no prison officers were injured.
On Tuesday night, three members of staff at the prison were assaulted and a female officer sustained a fractured jaw when more than 40 young offenders refused to return to their cells after a fight broke out at 1900 GMT.
The rioting at Moorland on Tuesday happened at the same time as about 60 offenders aged between 15 and 18 refused to return to their cells at Warren Hill in Woodbridge, Suffolk.
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