Claims a group of prison officers attacked an inmate in his HMP Liverpool cell are being investigated by police.
Mustafa Farooque was injured by up to three jail staff, his family said.
The 36-year-old is said to have a suspected fractured cheekbone and bruising to his stomach and back, along with grazing to his legs.
His sister and cousin say they have spoken with Farooque repeatedly since the incident, nine days ago.
Today, Merseyside Police confirmed they had received a complaint about the alleged assault, and an investigation was under way.
According to Farooque, he was suddenly attacked in his cell the day before Ramadan, when he was fasting.
He "curled up into a ball" as kicks were delivered to his body following orders to take off his trousers, he has claimed to his family.
He was seen by a doctor, but did not go to hospital, his family said.
Farooque, who comes from Bradford, is serving a four-year sentence for a violent road rage incident during which he twice threatened terrified motorists with an imitation handgun.
The Yorkshireman hurled racial abuse at an off-duty female police officer after he had driven his BMW erratically in queuing traffic.
On another occasion, he again brandished the black handgun, frightening a male driver who sped off and alerted the police.
His family said the 36-year-old had "turned over a new leaf" and "found religion" since those incidents in 2018.
Farooque, who has four children, aged between six and 16, was previously diagnosed with complex post traumatic stress order and suffered mentally after being shot in the chest in 2014, his family said.
Sister Zanned Farooque, 39, told the ECHO: "This incident in Liverpool has left me very distressed, and I've had to go into hospital for my own mental health.
"The family are very upset.
"When I was told, I broke down in tears."
His cousin has spoken to Merseyside Police about the incident, along with Ms Farooque, and they have instructed a solicitor.
A spokeswoman for Merseyside Police said: "We can confirm a report was received of an alleged assault at HMP Liverpool on Wednesday, 24 April.
"An investigation is ongoing to establish the full circumstances."
A Prison Service spokesman said: "It would be inappropriate to comment during an on-going police investigation.”
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Details of the alleged incident emerged days after the Council of Europe's Committee for the Prevention of Torture (CPT) released a report citing the "infliction of unjustified violence by staff on prisoners" at the Walton facility.
Name-checking Liverpool's main jail, and Wormwood Scrubs in London, out of the five prisons they visited last May, they said the alleged trend was "deeply concerning."