Four top Khmer Rouge leaders went on trial at Cambodia's UN-backed war crimes court on Monday for genocide and other atrocities during the hardline communist regime's reign of terror in the 1970s.
The case, described as the most complex since the Nazi trials after World War II, has been long awaited by victims of the totalitarian movement, which wiped out nearly a quarter of the population.
The four face charges including genocide, crimes against humanity and war crimes over the deaths of up to two million people from starvation, overwork, torture or execution during the Khmer Rouge's brutal 1975-79 rule.
All four suspects deny the accusations, including the genocide charges, which relate specifically to the murders of Vietnamese people and ethnic Cham Muslims.
Nuon Chea, the right-hand man of late Khmer Rouge leader Pol Pot, left the hearing after only half an hour in protest at the handling of the investigation and legal proceedings.
"I am not happy with this hearing," said Nuon Chea, 84, before returning to the detention facility. The defendants are allowed to be absent if they refuse to cooperate.
Former social affairs minister Ieng Thirith and her husband, ex-foreign minister Ieng Sary, were later excused on health grounds, leaving only one-time head of state Khieu Samphan in the dock for the duration of the hearing.
The accused, all in their late 70s or 80s, suffer from varying ailments and there are fears that not all of them will live to see a verdict.
The complex proceedings, expected to take years, are seen as vital to healing the traumatised nation's deep scars.
"This trial is very important to find justice for those who died and for the survivors," said Khem Nareth, 56, who lost his mother and brother under the regime.
At the end of the trial's first day, victims expressed satisfaction.
"We are pleased with the court's proceedings today. We are very happy," said 80-year-old Chum Mey, one of just a handful of people to survive a notorious Khmer Rouge prison.
Trial monitor Anne Heindel, a legal adviser to the Documentation Centre of Cambodia, said the hearing had provided "a snapshot" of what the trial would be like, "with health concerns being an overarching issue".
A main topic of debate on Monday was whether Ieng Sary's 1979 death sentence in absentia for genocide, in what was widely regarded as a sham trial, constituted double jeopardy.
Defence lawyer Michael Karnavas said the accused "should not be tried twice for the same crimes."
But international co-prosecutor Andrew Cayley argued the 1979 proceedings "did not even meet the most basic standards of a fair trial".
Hundreds of Cambodians travelled to the court for the opening of the trial. Parts of the proceedings were also broadcast on Cambodian television.
The initial hearing will continue over the next three days with more preliminary legal objections and talk of reparations for the nearly 4,000 victims taking part in the proceedings as civil parties.
Full testimony from the suspects, held at a purpose-built detention centre since their 2007 arrests, will not take place until late August at the earliest.
The trial is the culmination of years of preparation by the tribunal, which was established in 2006 after nearly a decade of negotiations between Cambodia and the United Nations.
In its historic first trial, the tribunal sentenced former prison chief Kaing Guek Eav -- also known as Duch -- to 30 years in jail last July for overseeing the deaths of 15,000 people.
The second case is more significant and complicated because it involves higher-ranking regime members who are refusing to cooperate, as well as many more victims and crime sites.
Led by "Brother Number One" Pol Pot, who died in 1998, the movement emptied Cambodia's cities and abolished money and schools in a bid to create an agrarian utopia before they were ousted from the capital by Vietnamese forces.