G4S has suspended nine members of staff from an immigration removal centre near Gatwick Airport, following a BBC Panorama undercover investigation.
The programme says it has covert footage recorded at Brook House showing officers “mocking, abusing and assaulting” people being held there.
It says it has seen “widespread self-harm and attempted suicides” in the centre, and that drug use is “rife”.
G4S said it is aware of the claims and “immediately” began an investigation.
The security firm said it had not been provided with recorded evidence, but added: “There is no place for the type of conduct described.”
The programme, to be aired on Monday, uses footage it says was recorded by a custody officer at the centre, which holds detainees facing deportation from the UK.
Panorama says it has seen “chaos, incompetence and abuse” at the centre, which it describes as a “toxic mix”.
It claims detainees who have overstayed visas or are seeking asylum in the UK can share rooms with foreign national criminals who have finished prison sentences.
It also alleges that some detainees have been held in the privately-run centre for “many months, even years.”
G4S said the staff suspensions were a “precaution” but it has reported the allegations to “the relevant authorities”.
“Such behaviour is not representative of the many G4S colleagues who do a great job, often in difficult and challenging circumstances,” Jerry Petherick, managing director for G4S’ custodial and detention services in the UK, said,
“Once we have seen the evidence and concluded the investigation, I will ensure that we take the appropriate action.
“We continue to focus on the care and wellbeing of detainees at Brook House.”
In response to the Panorama investigation, a Home Office spokesman said: “We condemn any actions that put the safety or dignity of immigration removal centre detainees at risk.
“We are clear that all detainees should be treated with dignity and respect and we expect G4S to carry out a thorough investigation into these allegations and that all appropriate action be taken.”
Brook House, which is operated privately by G4S on behalf of the Home Office, holds up to 508 adult male asylum seekers, illegal immigrants and foreign national offenders.
The centre was opened in 2009 but a year later was branded “fundamentally unsafe”.
A further report in 2012 found there were still “significant concerns”, but in 2013 inspectors saw sustained improvement.
The most recent report from HM Chief Inspector of Prisons, released in March this year, said some detainees had been held for excessive periods due to “unreasonable delays in immigration decision making”.
The report also described the residential units as “very closely resembling the conditions found in prisons”, saying problems were “exacerbated by poor ventilation and unsatisfactory sanitary facilities”.