A post-mortem examination showed Mr Mousa had at least 93 injuries
The British Army commanding officer of a detention camp punched a prisoner in the head, the inquiry into the death of Iraqi Baha Mousa has heard.
A soldier, identified as SO38, said he was "disgusted" to witness Lt Col Jorge Mendonca hit the detainee, verbally abuse him, and call him a "terrorist".
Mr Mousa, 26, died in Basra in 2003 while in British military custody.
Col Mendonca was cleared of all charges relating to the case at a court martial in 2007.
Mr Mousa, a hotel receptionist, was found dead with 93 separate injuries after being held in the custody of the Queen's Lancashire Regiment.
Only one soldier, former corporal Donald Payne, has been convicted in connection with his death - he pleaded guilty at a court martial to inhumanely treating civilians.
However, no-one has ever been punished for causing Mr Mousa's death.
In his witness statement released by the inquiry on Tuesday, SO38 said he saw Col Mendonca - the commanding officer or CO - hit a man who had been arrested during a search operation.
The statement said: "The CO, along with a number of other people, walked around the corner of the building and went straight up to the oldest one of the prisoners, where they were still standing under the shade."
He said Col Mendonca "then punched him in the side of the face" and swore at him, calling him a "terrorist".
"I do not know who else was with the CO at this time, but he did this in front of approximately 120 personnel of all ranks.
"I said to my men, 'Get round the corner, out of the way.' I did not want my men to see the CO's brutality towards a prisoner. By sending them out of the way, I was sending them a signal that I did not agree with the CO's actions.
"I was disgusted. A CO is supposed to lead by example."
On another occasion SO38 said he saw Col Mendonca "laughing" as an Iraqi was being punched in the back.
"I just could not believe what I had seen. It was beyond my comprehension," he said.
Col Mendonca - who left the Army in 2007 - will give evidence to the inquiry in February.
Witness SO38 also criticised the way British soldiers carried out searches of Iraqi homes, citing one instance where a building "had been rampaged".
"The troops had just started trashing the place," he said. "There were about eight people in each dwelling and the troops were throwing things on the floor, such as clothes, toys and paperwork.
"No concern was shown for the women and children in the buildings."
Giving verbal evidence to the inquiry, the ex-soldier said he had made a formal complaint that he was the victim of bullying, harassment and assault and had also lodged a civil claim in November 2007 for damages arising from his alleged treatment.
But he denied he was motivated by grudges or "had it in for" the Army, saying he had taken action for "justice" rather than compensation.
He said: "I want people who have abused their rank and status to pay with justice."
The inquiry continues.