Troops are accused of beatings and using electric shocks
Amnesty International is investigating claims that Iraqi prisoners of war were tortured by US and British troops.
A "substantial proportion" of 20 former Iraqi POWs interviewed by the human rights group said they had been maltreated by coalition forces in central and southern Iraq.
Some were civilians who were held on suspicion of being Iraqi militia, Amnesty said. One Saudi Arabian national claimed he was subjected to electric shocks.
The UK's Ministry of Defence said it had not seen any of the details of the allegations.
"All the prisoners held by the British were held under the terms of the Geneva Convention, and they were frequently visited by members of the International Red Cross," a spokesman said.
In one case we are talking about electric shocks being used against a man and in others people are being beaten for the whole night
The alleged mistreatment included "beatings with fists, with feet, also with weapons," Amnesty researcher Said Boumedouha said.
"In one case we are talking about electric shocks being used against a man and in others people are being beaten for the whole night and are still being kicked and their teeth broken. I think you would call that torture," he said.
The man claiming to have received electric shocks was believed to have been a Saudi man who had entered the country from Syria during the war and was suspected of being a volunteer for Saddam Hussein.
The findings follow Amnesty's first fact-finding mission in Iraq since 1993.
The human rights group said about 6,900 people were detained in Iraq while they were being assessed for POW status. Of these, about 1-2,000 remain in custody.
An Amnesty spokeswoman told BBC News Online the investigations into the treatment of PoWs were "at a very early stage".
Amnesty workers would be in Iraq for a number of months investigating a number of areas - including human rights abuses under Saddam Hussein - she added.