Cambodia's Khmer Rouge tribunal heard the first testimony from one of a few people to survive the horrors of the regime's main prison where around 15,000 others died.

Van Nath wept as he told how he was allowed to live only because he was put to work painting propaganda pictures of the communist movement's leader, Pol Pot, and that he was so famished he considered eating human flesh.

The 63-year-old, who is now one of Cambodia's top artists, was giving evidence at the trial of Duch, who is accused of overseeing the torture and extermination of thousands of people who passed through Tuol Sleng jail.

"The conditions were so inhumane and the food was so little," said Van Nath.

Duch, whose real name is Kaing Guek Eav, slumped in his chair and looked on while the artist recounted his arrival at Tuol Sleng, where he was photographed and then shackled with other prisoners.

"There were 20 or 30 of us in each row of shackles. The black clothes we were wearing, we were stripped of those clothes," Van Nath told the court, adding prisoners would rush to eat insects that fell from the ceiling.

"We only had three spoons of gruel for each meal. And the spoon was like a coffee spoon, it was not a normal rice spoon. I lost my dignity... Even with animals they would give enough food," Van Nath said.

Van Nath said prisoners shackled next to him died during his first month at the prison. So weak he needed assistance to stand and walk, he was summoned downstairs and thought his own death was imminent.

However, a prison official then told him he was needed to paint a large portrait of a figure he did not recognise -- the regime's leader, Pol Pot.

"I knew that if I did not paint very well, I would be in big trouble. I was so nervous," Van Nath said, calling it a "life and death situation."

Earlier in his trial for war crimes and crimes against humanity, Duch begged forgiveness from the victims of the 1975-1979 regime after accepting responsibility for his role in governing the jail.

But the 66-year-old has consistently rejected claims by prosecutors that he had a central role in the Khmer Rouge's iron-fisted rule and says he never personally executed anyone.