Feltham Prison and Young Offender Institution has been labelled an "unacceptably violent place" by inspectors.
It was covered in gang graffiti and witnessed on average two fights or assaults every day, some of which were "very serious", HM Inspectorate of Prisons found.
Chief inspector of prisons Nick Hardwick said: "Feltham as a whole is an unacceptably violent place. Despite excellent work in some cases, staff were unable to prevent a high number of very concerning incidents that carried a significant risk of serious injury."
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Police say he may have attacked more children and urged them to come forward.
The suspect walked up to a Tesco employee and shoulder barged him to the ground before punching him repeatedly in the face.
Up to 200 noisy motorists regularly meet in Imperial Way, Croydon attracting crowds of up to 400 people.