Seven staff members at a Kent young offenders centre have been suspended after filmed evidence of abuse was presented by the BBC.
The staff were working at the Medway Secure Training Centre in Rochester, managed by security firm G4S.
The allegations - uncovered by Panorama - involve unnecessary force, foul language and a cover-up.
G4S has written to the BBC to try to stop broadcast of the footage, arguing filming was unauthorised and illegal.
However, Paul Cook of G4S said he was "shocked and appalled" and was supporting a police investigation.
The claims relate to 10 boys aged 14 to 17.
The centre holds 56 children, mainly boys, aged 12 to 17. It is not allowed to take any more while the investigation takes place.
Mr Cook, managing director of G4S children's services, told BBC Radio Kent he was informed of the allegations on 30 December and referred them that day to the Medway Child Protection team and the police.
He said the BBC had referred to "staff hurting and inappropriately restraining young people, using foul and abusive language and failing to report... their actions".
Among the allegations uncovered by Panorama and now subject to investigation are that Medway staff:
- Slapped a teenager several times in the head
- Pressed heavily on the necks of young people
- Used restraint techniques unnecessarily - and that included squeezing a teenager's windpipe so he had problems breathing
- Used foul language to frighten and intimidate - and boasted of mistreating young people, including using a fork to stab one on the leg and making another cry uncontrollably
- Tried to conceal their behaviour by ensuring they were beneath CCTV cameras or in areas not covered by them
Mr Cook said the seven members of staff named in a Panorama document sent to him were immediately suspended.
Safeguarding had also been stepped up at the centre while the investigation takes place, he added.
The Howard League, which campaigns for penal reform, tweeted the allegations were "extremely concerning", adding it had legal clients at the centre.
Tweeting in a personal capacity, its CEO Frances Crook said she was pleased Panorama had investigated abuse in G4S homes.
She added: "@TheHowardLeague has represented children who've had serious injuries in G4S secure children's homes. It demands public concern."
Analysis: The BBC's home affairs correspondent Danny Shaw
Medway had a troubled start. Within months of opening in 1998 there'd been a serious disturbance, security breaches and mass staff resignations.
In recent years conditions appeared to have settled down. An Ofsted inspection report in September 2014 rated Medway as "good with some outstanding features". Inspectors said the centre was "orderly" and "calm"; young people were said to feel safe.
The evidence gathered by Panorama seems to paint a different picture and will raise questions about G4S's suitability for looking after some of Britain's most troubled, vulnerable and violent teenagers after it was stripped of its contract for managing Rainsbrook secure training centre in Northamptonshire in 2015.
There are also questions for those who regularly monitor Medway - did they have any concerns and did they alert the authorities?
Kent Police said in a statement: "All necessary safeguarding measures have been taken and inquiries are ongoing."
The Youth Justice Board said it had increased its monitoring of the centre and increased the presence of its independent advocacy service, delivered by Barnardo's.
Deborah Coles, who is the director of the charity Inquest, which helps people bereaved by a death in custody, said in any other setting the treatment "would be child abuse".
"That it took undercover filming to reveal the mistreatment of imprisoned children points to the culpable failure of monitoring and oversight by the YJB and G4S," she said.
"Following the deaths of two children in 2004 following the use of force, assurances were made that the culture and practices would be changed and yet the abuses continue.
"This points to a lack of accountability and culture of impunity.
"It is clear these institutions are incapable of reform and must be closed down."
The Panorama programme Teenage Prison Abuse Exposed will be broadcast on Monday, 11 January on BBC One at 20:30 GMT.