A monk pauses in a detention room at Tuol Sleng, a prison turned museum now under consideration for Unesco protection as a Memory of the World.

A small group of tourists on a recent Tuesday shuffled through the halls of Tuol Sleng, a dreaded prison under the Khmer Rouge that has become the most famous museum in Cambodia.

Through one old building to the next, the group moved, and in the eyes of some, tears built, as they scanned the mug shots of scores of Cambodians who had been photographed just before their executions. As the tourists passed the photographs of the dead, the beds where they had lain, and the shackles that had held them, a few had to flee the building, sobbing.

"Terribly shocked," one foreign woman said. "We all followed the events at that time, through media and television. And when you see the real place and you are a witness though the photographs and paintings and the torture instruments of the torture cell, it's difficult to imagine the horror and tragedy."

Still, she said, "I think it is important that this museum was made."

As many as 100 visitors pass through here a day, most of them foreigners.

"When you come here you feel like you just want to be silent and try to imagine," said Valerie de Robillard, a French visitor who come in Cambodia for the first time. "It is a dimension that is even more terrifying in this genocide."

A high school called Tuol Svay Prey until August 1976, Tuol Sleng became the Khmer Rouge's largest prison facility. In less than two and a half years, the prison held at least 12,000 Cambodians, and as many as 16,000, who were tortured and interrogated under suspicion of spying or disloyalty to the regime. Nearly all of them were later executed and dumped into mass graves on the outskirts of town, at a place called Choeung Ek, touted now by tour guides and taxi drivers as "the killing fields."

Kaing Kek Iev, the 65-year-old former chief of the prison better known by his revolutionary alias, Duch, is now set for trial under the Khmer Rouge tribunal. 

Duch's trial, which will be held by the beginning of 2009, will be the first in 30 years for leaders of the regime, which led one of the worst atrocities of the 20th Century.

The prison he once ran stands now as an icon of his regime's cruelty and will hold a center place in Duch's trial. The trial will show average Cambodians one of the clearest pictures of a regime that was shrouded in secrecy until the very end.

Tuol Sleng serves as a storehouse of evidence in that trial, having housed thousands of documents and photographs that testify to atrocities committed under the ultra-Maoist regime, under which as many as 2 million Cambodians died.

Only seven survivors of the prison have been positively identified, and only three of those survive. Records indicate as many as 177 were released, but these survivors have not been found.

Tuol Sleng is now under consideration as a Memory of the World site, its documents and photographs, the walls themselves, and even the Choeung Ek "killing fields" to be protected by Unesco.

"We need to serve justice for all victims, whether they died or survived," the director of the museum, Chhey Sopheara said.

Prisoners were routinely tortured here, their confessions against the regime extracted under the worst of circumstances: waterboarding, electric shock, burns, prolonged hanging.

The Vietnamese forces that ousted the Khmer Rouge, on Jan. 7, 1979, found the prison and recognized a need to preserve it. Tuol Sleng ceased being a prison and began its life as a museum on Aug. 19, 1979, following a trial in absentia of the regime's leaders, Khieu Samphan, Ieng Sary and Pol Pot. Pol Pot has died. Khieu Samphan and Ieng Sary have joined Duch in tribunal detention.

The Vietnamese left many of the rooms and cells as they found them. The museum is still inscribed by rusty coils of barbed wire, and visitors can walk past rows of cells just 1.2 meters wide. In a closet sit the old instruments of the Khmer Rogue: shovels, sticks, axes. Photographs of tortured children, handcuffed or chained, are on display on large wooden tables.

A requisite stop for most tourists in Phnom Penh, Tuol Sleng, known to the Khmer Rouge by its code name, S-21, is also visited by Cambodians who suffered from the regime.

"The prisoners were tied by their feet and hanged," said a man named Kreusnar, 26, whose father, older sister and uncle died under the Khmer Rouge and who came from Prey Veng province to visit the museum alone. "The Khmer Rouge prisoners were soaked in water jars or in basins. They had their nipples cut off. This torture to me seems fresh."