LONDON—A British judge was shown graphic pictures of dead and mutilated Iraqis who allegedly died at British hands as a public inquiry began Monday into claims that U.K. soldiers murdered and abused civilian detainees after a 2004 battle in southern Iraq.

The Al-Sweady Inquiry is investigating claims that up to 20 Iraqis were tortured and killed in May 2004 at a British military base in Iraq's Maysan province after a battle between British troops and insurgents. It is looking at some of the most serious allegations made against British forces during the war in Iraq and is named after one of the dead, 19-year-old Hamid Al-Sweady.

The inquiry will also examine whether five other Iraqis were mistreated at two British facilities.

Inquiry lawyer Jonathan Acton Davis said in his opening statement that there was a "stark dispute" between the evidence of the Iraqis and that of the British military about what happened on May 14 and 15, 2004.

The British military denies abuse and says 20 Iraqis were killed in battle after ambushing British troops. Several soldiers were decorated for bravery in the fierce engagement—dubbed the battle of Danny Boy after the checkpoint where it took place—which included the British army's first bayonet charge in two decades.

Iraqi witnesses, however, say some of the Iraqi men were taken from the battlefield alive.

"The Iraqi witnesses say that the evidence points to there having been a number of Iraqi men taken into (Britain's Camp Abu Naji) alive by the British military on May 14, 2004 and who were handed back to their families dead the next day," Acton Davis said.

The inquiry was shown gruesome pictures of some of the dead Monday. Acton Davis said the death certificates recorded injuries including gunshot wounds, broken bones, signs of torture and mutilation, including missing eyes. One man's penis was missing.

The government ordered the inquiry after Britain's High Court ruled that an earlier Royal Military Police investigation into the killings had been inadequate. That investigation found no evidence of wrongdoing by British troops.

The inquiry was ordered more than three years ago, but the hearings had to await a lengthy police investigation to gather evidence.

The inquiry, led by retired High Court judge Thayne Forbes, is expected to last a year and hear from hundreds of witnesses, including several Iraqis who will travel to London to give evidence.

The inquiry is opening just before the 10th anniversary of the U.S.-led invasion of Iraq in March 2003. Some 120,000 British troops served in the invasion and the subsequent occupation.

Allegations of abuse dogged the six-year British deployment, which ended in 2009. The most infamous was the case of hotel receptionist Baha Mousa, whose death in a detention facility outside the city of Basra led to the first conviction of a British soldier under international war crimes legislation.

Britain has paid more than 15 million pounds ($23 million) to settle more than 200 abuse cases.

Britain has already held an inquiry into the death of Mousa, as well as a separate wide-ranging public inquiry into the Iraq war, which is due to report later this year.