Prison officers have been injured during violence that broke out at a young offenders institute.
About 14 staff at Feltham Young Offender Institute in west London were treated in hospital after the disturbance at lunchtime on Friday.
No-one was seriously injured. It is not known how many youngsters were involved in the incident which was brought under control within 25 minutes.
The Prison Service said it was "quickly resolved by brave prison officers".
About 500 boys and young men aged between 15 and 21 are serving sentences at Feltham Young Offender Institute.
A Prison Service spokesman said: "An incident at Feltham Young Offender Institute was quickly resolved by our brave and diligent prison officers.
"We will never tolerate violence against our staff and will punish those involved."
The Ministry of Justice (MOJ) said there were no serious injuries, and the matter was brought under control "within 25 minutes".
An inspection report published in June identified soaring levels of violence and self-harm, high use of staff force, poor care and long periods of lock-up in cells in Feltham A, which holds offenders aged 15-18.
It prompted the government to halt placements at the unit.
The MoJ said there had been a sustained reduction in levels of assaults on staff, self-harm and use of force between the start of May and the end of August, but there are no plans to return the establishment to full capacity.