A UN-backed tribunal in Cambodia is holding its first hearing in the trial of four former top Khmer Rouge leaders.
The defendants include the "number two" in Pol Pot's regime, Nuon Chea. They face charges of genocide and crimes against humanity over the deaths of up to two million Cambodians in 1975-79.
They all deny the accusations, and the trial is likely to last for years.
Last July, former Khmer Rouge member Kaing Guek Eav, known as Comrade Duch, was jailed for 35 years.
But because of time already served and compensation for a period of illegal detention, Duch - the former head of a notorious prison where some 15,000 died - will be free in 19 years.
Led by Pol Pot, who died in 1998, the Maoist Khmer Rouge regime was ousted from power by Vietnamese forces in 1979.
The four defendants appeared at the initial hearing at the Extraordinary Chambers in the Court of Cambodia (ECCC) in Phnom Penh on Monday morning.
Besides Nuon Chea, they include former head of state Khieu Samphan, former foreign minister and international face of the regime Ieng Sary, and his wife Ieng Thirith, who was minister for social affairs.
The four showed no emotion as opening statements were read out before the court and a packed gallery, in proceedings screened on national television.
Moments later, Nuon Chea - who was dressed in a ski hat and sweatshirt - complained he was not well and felt cold and left the courtroom.
"I'm ready to come back when the court discusses my requests," he said.
The hearing will run for a maximum of four days, and no evidence will be given. Instead, the hearing is expected to focus on witness and expert lists and preliminary legal objections.
The trial proper is expected to open later this year, possibly in September.
"There hasn't been a case as large and complex as this since Nuremberg," international co-prosecutor Andrew Cayley told the AFP news agency in a recent interview, referring to the historic Nazi trials after World War II.
The head of the Cambodian Center for Human Rights said the start of the second case was a "cathartic moment".
The crimes "remain ingrained in Cambodia's collective psyche. I hope that this trial... provides all victims with some sense of justice, however delayed that justice may be", Ou Virak said in a statement.
Theary Seng, who lost both her parents to the Khmer Rouge, told the BBC: "This is the heart of the matter - this is the case that we have been anticipating for many years, since the operation of the court in 2006.
"It will allow a lot of information to surface which will help to chip away at the repeatedly-asked questions of why did it happen?"
Theary Seng is one of almost 4,000 civil parties to the case - victims who will have a voice in court alongside the prosecution and defence.
At least one of the defendants - Ieng Sary - is expected to argue that he should not be on trial at all, the BBC's Guy De Launey in Phnom Penh says.
The former foreign minister received a royal pardon 15 years ago as part of the deal which produced the final surrender of the Khmer Rouge.
The defendants are all in or near their 80s and some have been in bad health, so there is a real danger that not all of them may live to see the end of the trial, our correspondent adds.
The suspects have been kept in detention since their arrests in 2007.
Parts of court proceedings will be broadcast on TV, but hundreds of people from all over Cambodia are still expected to travel to the court to see the accused.
The current Cambodian government has repeatedly opposed efforts to widen the tribunal's inquiries, and insisted that there should be no further trials after that of the four leaders.