In London, authorities said the two commandos were released after negotiations. But the BBC quoted British defense officials as saying a wall was demolished when British forces went to "collect" the men.

Iraqi security officials on Monday variously accused the two Britons they detained of shooting at Iraqi forces or trying to plant explosives. Photographs of the two men in custody showed them in civilian clothes.

When British officials apparently sought to secure their release, riots erupted. Iraqi police cars circulated downtown, calling through loudspeakers for the public to help stop British forces from releasing the two. Heavy gunfire broke out and fighting raged for hours, as crowds swarmed British forces and set at least one armored vehicle on fire.

Witnesses said they saw Basra police exchanging fire with British forces. Sadr's Mahdi Army militia joined in the fighting late in the day, witnesses said. A British military spokesman, Darren Moss, denied that British troops were fighting Basra police.

Another Western military spokesman in Basra confirmed "an ongoing disturbance" in the city on Monday but said Iraqi and British forces were working together to quell it.