One of the few survivors of the Khmer Rouge's main torture centre wept at a UN-backed tribunal today as he recounted the conditions at the prison where 16,000 people were tortured before execution.

Vann Nath, 63, escaped execution because he was an artist and took the job of painting and sculpting portraits of the Khmer Rouge leader Pol Pot. His special status did not spare him misery.

"The conditions were so inhumane and the food was so little," Vann Nath told the tribunal, tears streaming down his face. "I even thought eating human flesh would be a good meal."

Vann Nath said he was fed twice a day, each meal consisting of three teaspoons of rice porridge.

"I lost my dignity," he said. "They even gave animals more food."

The testimony came at the trial of Kaing Guek Eav, better known as Duch, who headed the S-21 prison in Phnom Penh from 1975-79. Up to 16,000 men, women and children were tortured under his command and later taken away to be killed. Only 14 people, including Vann Nath, are thought to have survived.

Duch, 66, sat silently in his chair and watched Vann Nath closely as he spoke. Duch is charged with crimes against humanity and is the first of five defendants scheduled for long-delayed trials by the UN-assisted tribunal.

Duch has previously testified that being sent to S-21 was tantamount to a death sentence and that he was only following orders to save his own life.

Vann Nath said he was arrested on 30 December 1977 at his home in north-western Battambang province where he worked as a rice farmer. He was accused of trying to overthrow the Khmer Rouge and of being an enemy of the regime – a common accusation against prisoners. He arrived at S-21 on 7 January 1978 and was kept there until the regime collapsed about a year later.

Prisoners were kept shackled and ordered not to speak or move, Vann Nath told the court.

"We were so hungry, we would eat insects that dropped from the ceiling," he said. "We would quickly grab and eat them so we could avoid being seen by the guards."

He said prisoners ate their meals next to dead bodies and "we didn't care because we were like animals".

The regime's extreme policies caused the deaths of an estimated 1.7 million people nationwide by execution, overwork, disease and malnutrition.

Most prisoners were tortured into giving fanciful confessions that suited the Khmer Rouge's political outlook, though they generally had been loyal members of the group.

Duch is the first senior Khmer Rouge figure to face trial and the only one to acknowledge responsibility for his actions. Senior leaders Khieu Samphan, Nuon Chea, Ieng Sary and Ieng Sary's wife, Ieng Thirith, are all detained and likely to face trial in the next year or two.