PHNOM PENH (AFP) - Cambodia's UN-backed war crimes court on Monday opened the trial of three top Khmer Rouge leaders more than three decades after the country's brutal "Killing Fields" era. Judge Nil Nonn said the court "declares opened the substantive hearing" in the case against "Brother Number Two" Nuon Chea, ex-head of state Khieu Samphan and former foreign minister Ieng Sary. The regime's three most senior surviving members deny charges of genocide, war crimes and crimes against humanity over the deaths of up to two million people during the communist movement's 1975-1979 reign of terror. "It's a major milestone that finally this trial has started," said court spokesman Lars Olsen. "Many people never thought it would happen." Missing from the courtroom was fourth accused Ieng Thirith -- the regime's "First Lady" and the only female leader to be charged by the court -- after she was ruled unfit for trial last week because she has dementia. Judges have ordered her release, but she remains locked up while an appeal by the prosecution is considered, which is expected to take two weeks. Hundreds of Cambodians, including monks, students and regime survivors, packed the court's public gallery on Monday for the first of four days of opening statements in the landmark case, seen as vital for providing some justice for the still-traumatised nation. "I feel very happy. I came here because I want to know the story and how it could have happened," said 75-year-old farmer Sao Kuon, who lost 11 relatives under the Khmer Rouge. Parts of the proceedings will also be broadcast live on television. Led by "Brother Number One" Pol Pot, who died in 1998, the communist Khmer Rouge wiped out nearly a quarter of the Cambodian population through starvation, overwork and executions in a bid to create an agrarian utopia. Owing to fears that not all of the accused, who are in their 80s and suffer from varying ailments, will live to see a verdict, the court recently split their complex case into a series of smaller trials. But during the opening statements the prosecution and the defence will address all of the accusations. Of the accused, only Khieu Samphan has indicated he will cooperate, telling the court in June that while he was not "fully knowledgeable" about everything that happened, he would help to find "the truth". Nuon Chea walked out of the June hearing after telling the court he was "not happy". His lawyers say he will make opening remarks on Wednesday, but it is not known whether he will answer questions. And Ieng Sary, who was frequently the only point of contact between the secretive communist regime and the outside world, told the court last month he does not intend to testify. The case against the three is the court's second and most important after it sentenced former Khmer Rouge prison chief Duch to 30 years in jail last year for overseeing the deaths of 15,000 people. The case is now under appeal with a verdict expected on February 3. The new trial comes at a critical time for the perennially cash-strapped tribunal, which has been mired in controversy over its handling of two possible new cases against five lower-ranking cadres that are strongly opposed by the government. The government has denied interfering but Prime Minister Hun Sen -- himself a former Khmer Rouge cadre -- has made it clear he wants the court's work to end with case two, even saying last year that more trials were "not allowed".