Head of the British army, Gen Sir Mike Jackson, has said he "condemns utterly" abuse, as three soldiers face charges of mistreating Iraqi civilians.
The court was shown 22 photos depicting alleged abuse
Nine charges against the Royal Regiment of Fusiliers soldiers include forcing prisoners to simulate sex acts at a Basra aid camp in May 2003.
L/Cpl Darren Larkin admitted one charge of assaulting an unknown man at the camp but denied another charge.
Two other soldiers pleaded not guilty at the court martial in Germany.
Cpl Daniel Kenyon, 33, and L/Cpl Mark Cooley, 25, from Newcastle upon Tyne, entered not guilty pleas at the court martial in Osnabruck.
The court martial, which is expected to last three to four weeks, comes just days after a US soldier was sentenced to 10 years in prison for abusing prisoners at Abu Ghraib jail, near Baghdad.
Spc Charles Graner was regarded as the ringleader at the centre of an abuse scandal at the jail.
BBC correspondent Paul Adams said it was inevitable that some people in the media would describe the case involving the UK soldiers as "Britain's Abu Ghraib".
But it was important to stress "at this extremely early stage" in proceedings that nothing heard so far pointed to "an institutional quality" that was a key aspect of the Abu Ghraib affair, he said.
At the court martial William England, representing L/Cpl Larkin, 30, from Oldham, Greater Manchester, said his client was ashamed of the offence to which he had pleaded guilty.
He said: "He knows he has brought shame on his proud regiment, his name and his family."
Sir Mike would not comment on the case directly, or on 22 photographs depicting the alleged abuse of Iraqi prisoners which were shown to the court.
Sir Mike Jackson would not comment on the case
But he stressed that of the 65,000 British troops who had served in Iraq, only a "small number" had been accused of any mistreatment.
The general said the court martial's outcome would be studied to see if it raised any further issues for the Army.
Sir Mike said: "I have every confidence in the military investigative and judicial system," he added.
The photographs released by the court martial include a picture of two naked Iraqi men simulating anal sex with their thumbs raised to the camera and a photograph of two Iraqi men simulating oral sex.
The offences the three soldiers are accused of are alleged to have taken place at the aid camp, known as Camp Bread Basket, on or around 15 May 2003, weeks after coalition troops had ousted Iraqi dictator Saddam Hussein's regime.
The trio, who face a total of nine charges between them, are accused of abusing looters who were being detained at the camp in southern Iraq after attempting to steal powdered milk and food.
Lt Col Nick Clapham, prosecuting, told the hearing the camp's commander Maj Dan Taylor had ordered that looters should be "worked hard".
This breached the Geneva Convention, Lt Col Clapham said.
"The order to work by Major Taylor was an unlawful one but, even though the order was unlawful, had the defendants done no more they would not face the charges they face today," he added.
Evidence of the alleged abuse came to light when a set of photographs were left for processing at a shop in Tamworth, Staffordshire.
A developer called police because she was "disturbed by the content".
"It cannot be said that these photographs are of incidents that are anything other than shocking and appalling," Lt Col Clapham told the court.
The charges also include two relating to the alleged placing of a detainee on the forks of a forklift truck.
If found guilty, the trio, who are stationed at a British army base in Osnabruck, northern Germany, could be jailed and discharged from the Army.
They are being tried before Judge Advocate Michael Hunter and a panel of British officers.
The case was adjourned until Wednesday morning.