Two workers at a hospital have been suspended after a man with bipolar disorder claimed he was "medically, mentally and physically abused" there.
Hugo Allen, from Derby, was taken to Cygnet Hospital Kewstoke, near Weston-super-Mare, for six weeks in May.
He claims that while in solitary confinement, staff did "horrible things" like wash his face in urine.
The hospital said it was working with the Care Quality Commission (CQC) and police, who are also investigating.
Mr Allen, who says his condition makes him prone to delusions, was diagnosed with bipolar disorder in 2017.
The 23-year-old had been sectioned on two other occasions before arriving at the Cygnet in May.
His family contacted health professionals after his behaviour became "manic" and a doctor decided he should stay at a mental health facility.
Mr Allen was told the only hospital with a bed available was in Somerset, three hours from his home.
He claims staff would confuse him, including by telling him he was a vampire and that his parents worked at the hospital.
"They would do all of this confusion and abuse to get me to lash out and get angry so they could put me in solitary confinement, where they would do horrible things like wash my face in my own urine," he alleged.
On several occasions, Mr Allen claims he was punched in the face and body by hospital staff while they were restraining him.
He also alleges he was left with a black eye and a bruised face on one occasion.
Mr Allen's mother, who did not want to be named, told the BBC she went to visit her son at the hospital less than a week after he was sectioned.
She said he was heavily "drugged up" and was the worst she had ever seen him.
She said the hospital would not keep her updated on her son's condition and on one occasion, after driving 150 miles to the facility, she says she was refused access to see him.
The one time she spoke to her son on a video call, she said he had a bruise on his face.
Avon and Somerset Police confirmed it was investigating allegations a man was assaulted by staff while a patient at the hospital between 20 May and 1 July.
"Steps have been taken to safeguard the complainant and other patients at the hospital while the police investigate," a spokesman said.
Cygnet Hospital Kewstoke was last inspected by the Care Quality Commission (CQC) in February 2019, where it was rated "good".
In response to Mr Allen's claims, a hospital spokesman said: "The safety and care of our service users is of paramount importance and reports of concern are taken very seriously.
"In line with our safeguarding procedures, we have suspended two members of staff while this matter is investigated thoroughly."
He refused to comment further due to the ongoing police investigation.