Cambodia's former Khmer Rouge President Khieu Samphan (L) and former Khmer Rouge leader "Brother Number Two" Nuon Chea sit at the Extraordinary Chambers in the Courts of Cambodia (ECCC) as verdict is delivered on the outskirts of Phnom Penh August 7, 2014. [Photo/Agencies]

PHNOM PENH - The United Nations war crimes tribunal in Cambodia convicted two aging former top leaders of the Democratic Kampuchea, or known as Khmer Rouge regime, of atrocity crimes against humanity and sentenced them to life in prison, according to a verdict pronounced by tribunal's president Nil Nonn on Thursday.

The two convicts are Nuon Chea, 88, the chief ideologue of the Communist Party of Kampuchea, also known as "Brother Number 2", and Khieu Samphan, 83, the regime's former head of state.

The duo is among the few surviving top leaders of the regime that was responsible for the deaths of an estimated 1.7 million people from starvation, overwork, torture, execution, and massacre during the regime from 1975 to 1979.

"The trial chamber finds the accused Nuon Chea and Khieu Samphan guilty of crimes against humanity of extermination, murders, political persecution, and other inhumane acts comprising forced evacuations and attacks against human dignity committed between 1975 and 1977," said the verdict read by Nil Nonn, president of the tribunal, formally known as the Extraordinary Chambers in the Courts of Cambodia (ECCC).

"The chamber sentences the accused Nuon Chea and Khieu Samphan to life imprisonment," the verdict said.

The two ex-top Khmer Rouge leaders appeared in the courtroom to hear the pronouncement of the verdict, which was live broadcast on local television channels and radios.

The verdict came after a two-year trial of the two former leaders began in November 2011 and ended in October 2013.

In their closing statements in October 2013, both Nuon Chea and Khieu Samphan denied that they were involved in alleged crimes against humanity during the regime.

"I had not involved in any crimes during the Democratic Kampuchea as alleged by the co-prosecutors and the co-prosecutors have not had enough evidence to prove their allegations against me, " Nuon Chea said at that time, blaming the deaths and executions of people during the regime to the betrayal by local cadres and regional leaders.

Khieu Samphan said at that time that he was innocent.

"I was among the powerful persons during the regime, but I was not as powerful as them," he said.

Defense lawyers for the two convicts said in a press conference after the judgment that they would appeal against the verdict.

Cambodian Deputy Prime Minister and Cabinet Minister Sok An said he was personally contented with the judgment.

"The pronouncement of the judgment today is clearly a testament of the success of this tribunal, particularly it has proved that this tribunal is competent and fully capable of conducting a fair trial in accordance with an international standard," he told reporters after the verdict delivery.

ECCC's spokesman chief Dim Sovannarom said it was a milestone in Cambodian history.

"After more than 35 years, now the justice has come for the victims," he said. "This was the first time ever that former Khmer Rouge senior leaders were judged by the court for crimes against humanity."

He said about 800 people, including 200 media members, attended the court on the outskirts of Phnom Penh, the capital of Cambodia, to watch the verdict.

Besides the case, the two convicts are also facing the second trial for alleged genocide of Muslim Cham and Vietnamese ethnic minorities, forced marriages and rape, treatment of Buddhists, and internal purges during their regime.

Launched in 2006, the United Nations-backed tribunal is seeking justice for the victims during the regime. As of last year, the tribunal had spent 204.6 million U.S. dollars.

In March last year, another defendant Ieng Sary, former foreign minister of the regime, died on trial at the age of 87 due to illness, while the case against his wife Ieng Thirith, 82, ex- minister of social affairs during the regime, was suspended in September 2012 after the court ruled dementia left her unfit to stand trial.

In 2012, the tribunal's supreme court sentenced ex-chief of Tuol Sleng prison Kaing Guek Eav to life in jail for overseeing the deaths of around 15,000 people during the regime.