You can see the building from a distance, a sinister grey shape blackened by the flames of looting.

A week ago no Iraqi in his right mind would have gone near the place. Now they are descending on it in their thousands, waving at our car, pointing, shouting and pouring in through the gates and over the walls.

Some carry fragments of photographs or torn lists of names in Arabic. Others are bringing shovels and hammers. They are pointing at the ground and holding their hands up before them, crossed at the wrists, as if handcuffed.

“My brother, my brother!” one man shouts in English. He stamps on the ground.

Inside the compound, where so many Iraqis have been shackled, tortured and killed, an extraordinary spectacle is being enacted. In the caged cells and on the dusty ground, people are digging frantically. A huge mechanical excavator has been commandeered and is flailing its digging arm in the central courtyard.

In the cells, men with picks are chipping away at the concrete floor. “There have been voices at night, everyone has heard them, all kinds of voices,” one man says. “Listen, listen! All of you, shhh! Shhh.”

The shouting and hammering falls away, and I press my ear to the floor.

There are two or three thousand people here and they are convinced of a remarkable thing: that, beneath here, hundreds of their friends, sons and brothers are entombed in a secret underground cell, abandoned there by the Iraqi security police.

It is a week since the security police fled from British troops entering Basra. Anyone trapped down there must be close to death from hunger and dehydration. The people of Basra hear the voices and plead with the soldiers to come and save their loved ones. But there is no evidence that any of it is true.

At half a dozen spots around the city, similar scenes have taken place over the past few days — a vast crowd, scrabbling at the earth in response to cries and voices.

The bemused squaddies lumber out to investigate in their Warrior armoured vehicles, but no underground dungeon has been found. And as I strain my ears against the floor, I hear nothing.

One man claims to have been held below the police HQ himself, for three weeks in 1999. But he does not know where the concealed entrance is because he was blindfolded. How then can he be sure that it was here, rather than in some other cell? Another holds up a face in a photograph, found in the abandoned headquarters, of a man who disappeared in 1991.

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