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 Maoist group\'s reign of terror in the late 1970s. The case, described as the most complex since the Nazi trials after World War II, has been long awaited by survivors of the regime, which wiped out nearly a quarter of the population. The elderly defendants, including \"Brother Number Two\" Nuon Chea and former head of state Khieu Samphan, looked frail as they sat in the dock. They 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. The genocide charges relate specifically to the murders of Vietnamese people and ethnic Cham Muslims. All four suspects, who also include ex-foreign minister Ieng Sary and his wife and one-time social affairs minister Ieng Thirith, deny the accusations. \"I am not happy with this hearing,\" 84-year-old Nuon Chea, wearing his trademark sunglasses, said before abruptly leaving the courtroom and returning to the detention facility with the judge\'s permission. The defence lawyer for Nuon Chea, seen as the movement\'s chief ideologue, argued that the investigation into the case had lacked transparency and suffered from government interference. \"That judicial investigation was so unfair that the case should be stopped,\" Michiel Pestman said. He said Nuon Chea was insisting that all his proposed witnesses -- believed to number in their hundreds -- should be heard during the trial. 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. \"I want the court to jail the four leaders for life,\" he added. The initial hearing is scheduled to last four days and will focus on expert and witness lists and preliminary legal objections. 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. Hundreds of Cambodians travelled to the court on Monday to see the four in the dock. Parts of the proceedings were also broadcast on Cambodian television. 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. Concerns over the health of the accused, aged 79 to 85, also hang over the proceedings. They suffer from varying ailments and there are fears that not all of them will live to see a verdict. Led by \"Brother Number One\" Pol Pot, who died in 1998, the communist regime 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. The start of the trial comes as the court faces criticism from observers and victims for allegedly failing to properly investigate two more cases against five lower-level Khmer Rouge suspects. Cambodian Prime Minister Hun Sen, himself a former cadre, is strongly opposed to further trials beyond the second case, saying they could destabilise the country, and observers believe they are likely to be abandoned.