Officers have contained a disturbance at Aylesbury Young Offenders' Institute, the Prison Service said.
Specially-trained prison officers and police were called in to deal with an outbreak of "concerted indiscipline" at the prison in Bierton Road, Aylesbury.
Thames Valley Police said its officers were sent in to secure the perimeter.
The incident lasted almost six hours. A Prison Service spokesman said there were no reports of injuries and no hostages had been taken.
He said the incident began at 1115 GMT and ended when Prison Service staff "intervened" at 1700 GMT.
HMP Aylesbury holds up to 435 young men on the site of a Victorian prison and former borstal.