Baha Salim Musa was arrested in the southern Iraqi city of Basra last month.
But after being held for four days, his father was asked to identify his body.
"His face was covered in blood, his nose broken, and the skin on his face was torn. There were bruises on his neck and all over his body," Baha's father Dawood told Reuters news agency.
"One wrist was broken and the flesh exposed where handcuffs had been pulled too tight. A sergeant confirmed that a rope had been put round his neck."
Dawood is convinced his son was tortured to death, and wants to know what happened.
The British military, which is responsible for Basra since the war on Iraq ended, says it is investigating the issue.
"Those suspected of any crimes will be tried, and if found guilty, punished under the laws of the United Kingdom"
Lieutenant Colonel David Amos
acting commander of UK forces
"Seven men were arrested during a planned operation on the 14th of September, and one subsequently died in custody," a British spokesman said, adding the Special Investigations Branch of the Royal Military Police was investigating Baha's death.
Lieutenant Colonel David Amos, acting commander of British forces in the Basra and Maysan region, said this week that "those suspected of any crimes will be tried, and if found guilty, punished under the laws of the United Kingdom".
Dawood had just dropped off his son at the Hotel Al-Haithum, where Baha worked as a receptionist, when the British raid began. Raids by occupation troops to seize weapons or detain
suspects are common across Iraq.
"The safe was broken into by the soldiers and the money inside seized. They found three rifles and two pistols – needed for the security in the hotel," he said.
"Then they called for reinforcements. That was when I saw my son and six of his colleagues lying face down on the floor with their hands behind their heads."
His death left his two young children orphaned – Baha's wife died just before the war. Baha had also looked after four of his nephews after their father died.
UK soldiers have had an easier
time than their US counterparts
"My son did not belong to any political party," Dawood said. "He didn't even read the papers. He always said he wasn't interested in the war."
Kefa Taha, who was in charge of the generator at the hotel, was also arrested in the raid. He is currently in a critical condition in Basra's Shaiba hospital and unable to talk.
British army hospital records say that Taha, 44, was admitted with "renal failure, rhabdomyolysis and severe bruising to his upper abdomen and the right side of his chest".
"We know that only a few of the soldiers are bad. The soldiers and officers we have dealt with since the death have been very excellent," Baha's brother Allah said.
"But maybe this has happened before and gone unpunished. We don't want anyone else to suffer like us."