The trial of three top surviving Khmer Rouge leaders accused of taking part in the Cambodian genocide that killed nearly 2 million people in the 1970s began on Monday.

The defendants are Nuon Chea, known as Brother Number Two, second-in-command under Pol Pot’s Communist regime, former head of state Khieu Samphan, and former foreign minister Ieng Sary. The men, all in their 80s, have denied the charges against them.

The wife of Sary, former social affairs minister Ieng Thirith, was ruled unfit to stand trial due to poor health. She faced similar charges.

The three are accused of genocide, crimes against humanity, and torture for the roles they played in the regime. Since the Cambodian genocide is considered one of the worst in history, Monday’s case is considered one of the most significant due to the severity and magnitude of the crimes the defendants are charged with.

Following in the Chinese Communist Party’s footsteps, the Khmer Rouge attempted to emulate Mao Zedong’s regime by killing off intellectuals, community leaders, monks, and so-called enemies of the state, while forcing city residents to work on community farms.

Between 1.7 and 2.5 million people died under the regime, out of a population of 8 million. Many Cambodians died of starvation, exhaustion, or were simply executed and buried in mass graves at sites known as the “Killing Fields.”

About 1,000 people showed up to watch the proceedings, in which the prosecution detailed the crimes committed under the Khmer Rouge including forcing millions of Cambodians to endure torture and a range of inhumane conditions before they died.

“Democratic Kampuchea … was one of the most brutal and horrific regimes in modern history,” prosecutor Chea Leang said in conclusion, illustrating how the Khmer regime executed numerous Cham Muslims and Vietnamese from 1975 onwards.

“These crimes ordered and orchestrated by the accused were among the worst horrors inflicted on any nation in modern history,” Leang said, according to the Australia Broadcasting Corporation. She listed examples, including the regime forcing Buddhist monks to renounce their religion and get married or face execution.

Leang also told the court that security forces beat a two-year-old child to death against a tree and held a throat-slashing competition, according to the broadcaster.

After the Khmer Rouge collapsed in 1979, the prosecution for many of its senior-level officials has taken decades. The current Cambodian government, which is headed by a former Khmer Rouge official, only recently asked the United Nations to start up an internationally-recognized tribunal to prosecute former regime authorities. 

It was not until last year that notorious torture camp operator Kaing Guek Eav, known as “Duch,” was convicted by the U.N.-backed Extraordinary Chambers in the Courts of Cambodia (ECCC), and was the first person to be sentenced under the tribunal. 

Despite the delays in prosecuting the alleged war criminals, U.N. human rights chief Navi Pillay, while speaking in Geneva, called the hearing “another historic day for the people of Cambodia, many of whom have waited a long time to see the start of this trial, and who can at last begin to hear evidence of the atrocities committed all across the country over 30 years ago.”

“The survivors’ testimony will undoubtedly help a new generation of Cambodians to understand their history and add impetus to the international community’s efforts to prevent future mass crimes,” she added.

Pillay noted that the U.N.-backed tribunal has faced challenges, mainly in protecting its impartiality and integrity during the proceedings, following an independent investigation.

“It is essential that these concerns are squarely addressed as the Court moves forward,” Pillay added said. “Allegations of interference mar the credibility of any Court in the eyes of the public.”