A lawyer representing the inmates from Stafford prison said the Victorian low-security jail had a reputation of being "ruled by fear and brutality".
The allegations follow claims of brutal treatment of prisoners in London's Wormwood Scrubs prison, which are also now the subject of police investigations.
Up to 19 Stafford prison officers are accused of taking part in the assaults, although not all have been identified. Officers from Staffordshire CID are believe to have interviewed the inmates and some of the officers.
In the most serious case, a man in his early twenties claims he was attacked in a segregation unit by four officers. He told his lawyers that on one morning of last December he was taken to the unit for a strip search while his cell was being examined. He claims an officer wanted to look at his private parts but he refused. He was then allegedly placed in a restraint position.
In a statement to the lawyer, the inmate said: "I was thrown face down on the floor and my head hit on the floor. The officers then pushed my legs apart and I felt a hand on my backside and I started struggling. Next two officers stood with their boots on my calves to keep my legs still." He claims an officer then went behind him and sexually assaulted him.
"I was then placed in restraints and locks on my wrists and legs and they went out leaving me on the floor."
His lawyers have obtained photographs of severe bruising to his legs and the prison chaplain has reported a black eye. An inmate locked in a nearby cell has also said that he heard shouting from the alleged victim.
The inmate making the allegation has been transferred to another prison since the complaint was made.
In the second investigation, two male inmates at Stafford claim to have been attacked by five officers while their cell was being searched. One prisoner claims to have had bruising to his head and legs. They alleged that after fighting back, up to 15 officers took part in assaulting them.
Allison Leary, a prison rights advisor of AS Law, a legal firm in Liverpool, said: "It's an old Victorian prison. The general feeling is that the conditions are awful and its ruled by fear and brutality. The majority of inmates say there are certain officers they dread being on their wing."
The jail is a training prison for about 600 Category C risk inmates who have been sentenced to more than two years.
The cases are the latest claims of prison officer brutality. In March, eight officers from Wormwood Scrubs in London were suspended and reported to the police following allegations of systematic violence and racism against eight inmates. The police inquiry is continuing.
A Prison Service spokesman said: "The allegation of a sexual assault has been made by a prisoner against a member of staff at Stafford prison. The allegation is the subject of a police investigation. Some members of staff have been interviewed by police."
A spokeswoman for Staffordshire police would only say: "We can confirm that complaints have been made to us and they are being investigated by the police."Reuse content