An Iraqi security guard broke down in tears while telling a court martial how he was allegedly beaten by UK soldiers.
Baha Mousa died while in custody
Juwad Fayez said he was punched and kicked repeatedly for failing to stand with knees bent and arms outstretched.
Cpl Donald Payne has already admitted the war crime of inhumanely treating Iraqi civilian detainees.
The 35-year-old and six other soldiers denied all other charges relating to alleged abuse in which one person died in Basra, southern Iraq, in 2003.
Mr Fayez wept for several minutes while giving evidence about the alleged beating at the court in Bulford, Wiltshire.
He said it happened at a detention centre after he was arrested as a suspected insurgent.
He said he was punched and kicked repeatedly for failing to hold the "stress position", and beaten with a metal pole.
"I was beaten many times and you can see in my photos the sort of shape I was in."
Urged to calm down, Mr Fayez told the court through an interpreter: "How can I calm down?"
As well as being assaulted, he said he was humiliated by soldiers breaking wind on him and playing with his nipples.
Mr Fayez also broke down when he described how he heard another detainee, Baha Mousa, calling out he was going to die.
"He was screaming and saying 'Oh my god, I'm going to die, I'm going to die.'
CHARGES IN FULL
Cpl Donald Payne - manslaughter, inhumane treatment of persons, perverting the course of justice
L/Cpl Wayne Crowcroft - inhumane treatment of persons
Pte Darren Fallon - inhumane treatment of persons
Sgt Kelvin Stacey - actual bodily harm, alternatively assault
Warrant Officer Mark Davies - negligently performing a duty
Maj Michael Peebles - negligently performing a duty
Col Jorge Mendonca - negligently performing a duty
"He was screaming all the time.
"I heard the screaming many times."
Mr Mousa, 26, a hotel receptionist, was among a group of detainees arrested following a counter-insurgency operation.
He died while in custody.
Cpl Payne, L/Cpl Wayne Crowcroft and Pte Darren Fallon are charged with the inhumane treatment of persons, which is a war crime under the International Criminal Court Act (ICCA) 2001.
It is the first time British military personnel have been prosecuted under the act.
Cpl Payne was the first British serviceman to admit a war crime.
He denies manslaughter and perverting the course of justice.
The other six soldiers each deny the charges they face.