Tributes have been paid to Vann Nath, the artist who was one of only a handful of people to survive the Khmer Rouge's notorious Tuol Sleng prison, who died on Monday.
The 66-year-old died in the capital, Phnom Penh, after a long illness.
He was sent to Tuol Sleng in 1978 but survived because he was told to create artworks of Khmer Rouge leader Pol Pot.
He went on to paint images of torture he witnessed there and testified before the trial of Duch, the prison chief.
"Vann Nath has fought his entire life for justice to be done, for victims to be heard and for the truth to be known," said a statement from lawyers representing civil parties to the UN-backed genocide tribunal in Phnom Penh.
"His struggle is for all of us a model of integrity and hope."
Youk Chhang, of the Documentation Centre of Cambodia - which records human rights abuses under the Khmer Rouge, described him as a witness to history who "exhibited great strength in providing his testimony despite the horrific crimes he suffered".
'Why did I live?'
Born in 1946, Vann Nath trained as an artist. When the Khmer Rouge seized power in 1975 he was sent to a co-operative farm to work.
Declared an enemy of the regime in 1978, he was sent to Tuol Sleng prison, where at least 15,000 people are thought to have been killed.
There he was ordered to paint pictures of Pol Pot. He was one of less than a dozen people found alive at the prison by Vietnamese forces who ousted the Khmer Rouge in 1979.
"To this day I still don't know why I was allowed to live," he told the BBC in an interview two years ago.
He went on to record the torture he witnessed at the prison in his paintings, becoming one of Cambodia's most prominent artists. He also wrote a memoir of his time at Tuol Sleng.
And he was the first survivor to testify against former prison chief Duch at his trial in 2009.
Describing weeks of being shackled to other prisoners, constant hunger and thirst, he said: "I told myself I did not care any longer because I could die any time and I'd rather die than live in such conditions."
Duch was later convicted for war crimes and crimes against humanity.
Tribunal spokesman Lars Olsen said Vann Nath had given "a voice to victims both through his testimonies before the court and through his lifelong work at Tuol Sleng museum".
So far Duch is the only former Khmer Rouge leader to have been convicted by the tribunal.
Four more top leaders, including Pol Pot's deputy, Nuon Chea, are due to be tried in the coming months.