The mother of a severely-disabled man who is a patient at Muckamore Abbey Hospital described a seclusion room her son was placed in as "a dark dungeon".
The woman said she was horrified that in 2017 the room was being used for people with learning disabilities.
Families want a public inquiry to investigate allegations of physical and mental abuse at the hospital.
The Belfast Trust said it "apologised sincerely" for behaviour it said fell below professional standards.
In July, it emerged that 13 members of staff at the County Antrim hospital had been suspended by the Belfast Health and Social Care Trust following allegations of ill treatment.
At the time it said: "This regrettable and unacceptable situation in no way reflects the work of our 500 dedicated and professional staff who provide excellent care every day to the 80 patients in Muckamore."
The man who was secluded at the hospital's Psychiatric Intensive Care Unit (PICU) last year has complex disabilities including autism and epilepsy and cannot speak or feed himself.
His family says he has the mental capacity of a two year old, and was locked in the room for up to two hours more than once.
His mother, who has asked for her identity to be protected, said alarm bells started ringing as soon as she saw the room.
"I saw a room 12 by eight, with padded walls, and a leather chair sitting in the middle of it," she told BBC News NI.
"There was nothing to look at, no stimulation whatsoever.
"The room had no toilet, nor drinking facilities.
"All I thought of was my poor boy was in there with not even a drink," said the mother.
"In 2017 this was happening in our country."
'Knuckles rubbed on head'
CCTV footage from the PICU unit shows her son being punched in the stomach by a nurse.
The footage, taken over a three-month period, also shows patients being pulled, hit, punched, flicked and verbally abused by nursing staff.
Sources have told the BBC the CCTV footage shows some of the most degrading and cruel behaviour against vulnerable adults ever captured in the UK.
A father was visibly upset as he described what he had been told about his son's treatment.
"They swung him round by the arm for over a minute and then let him go and he fell. He has epilepsy, he's on medication," he said.
"They tipped him out of his chair. Knuckles were rubbed against his head and various other incidents that I still don't know about but that is on the CCTV footage.
"I only know a short resumé of the allegations. I have been advised they are horrendous.
"I feel so guilty for putting him in, but as a parent you thought you were doing the best thing for him.
"He was let down so badly by the system and so were others.
"It is deeply distressing for a parent to be aware that you have entrusted your son into care and it went so miserably wrong."
The Belfast Trust said the seclusion room was still used in emergencies, but its use was being reviewed.
An adult safeguarding investigation began in September 2017 following reports of inappropriate behaviour and the alleged physical abuse of patients by staff in the PICU and another ward in the hospital.
Each family affected will receive a report about the care and treatment of their child, and the BBC understands families are being offered the opportunity to view some of the CCTV footage from Muckamore.
A Serious Adverse Incident (SAI) report is due to be published, which the BBC understands will outline "catastrophic and systemic failures" involving senior management and nursing staff.
Its author, Margaret Flynn, headed up the review into the Winterbourne View abuse scandal in England.
The level of abuse at Muckamore Abbey is on a similar scale, sources say.
The PSNI confirmed last week it was investigating 132 potential criminal cases, but that figure is expected to rise: The BBC understands the scale of the investigation is unprecedented.
A specially commissioned panel is reviewing more than 90,000 hours of CCTV footage, which may trigger more investigative actions.
Since November 2017, 13 staff members have been placed on "precautionary suspension" after ill-treatment allegations.
They remain off duty, on full pay.
One senior manager is understood to have offered to retire early after allegations were made, and is currently off sick.
A senior nurse is also on long-term sick leave.
'She was black and blue'
One County Down mother, whose daughter was an in-patient at Muckamore, said she repeatedly voiced her concerns to management and the regulatory body the RQIA, but was ignored.
"I felt powerless that no matter what I did or who I spoke to, including the RQIA and health trust management, the same things kept happening over and over again," she said.
"I raised concerns about the continuous incidents. She was black and blue, being hit by other patients.
"She was trailed up the corridor by her hair but no one listened. She was failed, I was failed."
The woman raised her own concerns about the seclusion room after her daughter appeared traumatised and said she had been "put in jail".
The Belfast Health and Social Trust said it took the concerns raised by relatives extremely seriously, and had arranged to meet one of the families.
The RQIA said it followed up each case raised "in line with our powers".
"We are constantly working to improve out accessibility, so as people can bring their concerns to us in the knowledge action will be taken.
"The abuse, by its very nature, was hidden from management at this service, from visiting professional staff such as psychiatrists who were present on a daily basis."
Due to the complexity and scale of the investigation, the National Crime Agency has been asked to provide advice to the PSNI if requested, but has not been asked to play an active role.