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."
Those suspended were one female nurse, six detention custody officers, and two managers, who were all male.
Five other members of staff have also been placed on restrictive duties, the BBC understands.
A former G4S officer, who now works for the Home Office, has also been suspended.
The programme, to be aired on Monday, uses footage it says was recorded by former custody officer Callum Tulley 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 are failed asylum seekers can share rooms with foreign national criminals who have finished prison sentences.
These criminals, who are awaiting deportation, make up about half of the people in Brook House.
Because of the amount of time people are being held for, "there is a huge amount of frustration being built up," BBC home affairs correspondent Danny Shaw said.
He added the allegations raised "serious questions" about the company's capability to manage places like this in the future.
G4S said the staff suspensions were a "precaution" but it reported the allegations to "the relevant authorities".
Speaking to BBC Radio 4's World at One, the managing director for G4S's custodial and detention services in the UK said the alleged behaviour was "abnormal".
Jerry Petherick told the programme: "It's despicable, it's disgraceful. It's my job to drive the expected behaviours... [and] I take personal accountability and responsibility for doing that."
He said the detainees were "all vitally important individuals" and that he wished he had known about the allegations sooner.
Mr Petherick confirmed G4S would continue with its bid for the next contract for Brook House.
But the chairwoman of the all party parliamentary group on refugees said the contract "should be suspended and reviewed".
Labour MP Thangam Debbonaire told World at One: "These are people who are not criminals, they are being detained. And the use of detention, unfortunately, over the last few years has, by the Home Office, become a port of first resort rather than last resort as it is supposed to be."
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."
Past controversies and criticisms
G4S runs Brook House on behalf of the Home Office as well as working in a number of sectors, including technology, care and justice services, and cash transportation.
The firm has 585,000 employees across 100 countries worldwide but has also attracted controversies and accusations of mismanagement.
In 2014, it paid £109m for overcharging the Ministry of Justice for tagging offenders, while it also failed to employ enough security for the London 2012 Olympics.
Last year, criminal proceedings were launched against eight G4S staff for their treatment of young people at the Medway Secure Training Centre.
In 2010 - a year after opening - Brook House was branded "fundamentally unsafe". Three years later inspectors said they 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".
It described residential units as "very closely resembling" prisons, saying problems were "exacerbated by poor ventilation and unsatisfactory sanitary facilities".
A review of improvements made at the centre will begin on Monday.
Who are the detainees?
Brook House is currently home to 508 men - with the highest numbers coming from Pakistan, Albania, Nigeria, Afghanistan and India.
According to the Home Office, the majority of those held are failed asylum seekers or illegal immigrants waiting to be deported from the UK on organised charter flights.
Other detainees include foreign national offenders awaiting transfers and those who are considered too challenging to manage in less secure centres.
Brook House is one of 11 detention removal centres in England, which together took in 28,908 people last year - including 71 children.
During the year, 28,661 people left detention - of which 64% were held for less than 29 days, 18% for between 29 days and 2 months, and 11% for between two and four months.
Of the 1,848 (6%) remaining, 179 had been in detention for between one and two years, and 29 for two years or longer.
Watch Panorama - Undercover: Britain's Immigration Secrets - on Monday 4 September at 21:00 BST on BBC One and afterwards on BBC iPlayer.