Hundreds of detained immigrants claim to have been physically assaulted or abused by staff at privately-run detention centres, a charity says.
The Home Office says any staff breaching rules will be disciplined
Bail for Immigration Detainees (BID) told the BBC's Today programme many are afraid to speak out because they fear that would adversely affect their case.
BID says it also gets complaints about a lack of respect in the centres.
A Home Office spokesman said that any staff found not to treat detainees with respect and dignity could be dismissed.
One detainee - 24-year-old Suzan from Uganda, who is HIV positive and weighs six stone - says she was excessively restrained by four officers at the Colnbrook detention centre near Heathrow Airport, after she refused to hand over her walking stick to a nurse.
"Two were holding my arms, two were holding my legs and then they hit my head on the floor," she said.
"I was feeling pain and then they twisted my arms and pressed my head on the bed.
"I couldn't breathe and then I was shouting 'I can't breathe, I can't breathe' but they were just twisting it harder."
BID caseworker Anna Morvern said Suzan's was not an isolated case.
"We get enough calls to our helpline to be very concerned about abuse and violence going on in the detention centres.
"They're often very scared about reporting abuse or ill treatment to the authorities.
"They fear it will have a detrimental effect on their claim to stay here, if they're claiming asylum, or they fear reprisals, they fear further ill treatment."
Dr Frank Arnold, who examines detainees on behalf of the Medical Justice Network, said he had seen a number of injuries consistent with claims of assault.
"There are many, many cases of this kind of injury which have been reported to the police, and reported by our organisation, the Medical Foundation for the Care of Victims of Torture and by BID."
Police and the Home Office are both investigating the complaint made by Suzan, who was due to be removed to Uganda on Tuesday night before being granted a temporary reprieve.
The Home Office spokesman said that anyone found to have contravened rules on humanity and respect would face disciplinary action and possible dismissal.
Tom Riall, chief executive of the home affairs division of Serco, which runs Colnbrook, said staff there did "a difficult job".
"But we do it with care and decency and considerable respect for all of those in our charge.
"We only use physical restraint as a last resort and, on the occasions where we do use physical restraint, absolute minimum force is used."
The allegations involving Suzan were now being investigated by the police, he said.
"But as I understand it, the detainee concerned was both kicking out at and attempting to bite members of staff and, under those circumstances, it was considered necessary to use physical control and restraint."