A UN-backed tribunal in Cambodia has sentenced two former Khmer Rouge leaders to life in prison for crimes against humanity related to their role in the country's 1970s terror.

Thursday's verdict follows a two-year trial of "Brother Number Two" Nuon Chea and former head of state Khieu Samphan - the most senior surviving ex-Khmer Rouge officials.

They were accused of playing key roles in a regime that oversaw the deaths of two million people during the "Killing Fields" era from 1975-1979.

Nuon Chea, 88, wearing his trademark sunglasses, sat in a wheelchair in the dock next to Khieu Samphan, 83, as proceedings got under way.

The tribunal's chief judge Nil Nonn finally asked both men to rise for the verdicts but the frail Nuon Chea said he was too weak to stand and was allowed to remain seated.

There was no visible reaction from either of the accused as the judge said both men were found guilty of crimes against humanity, forced transfers, forced disappearances and attacks against human dignity and sentenced to life imprisonment.

Verdicts to be appealed

The lawyers of the two vowed to appeal the verdicts and the sentences.

"We will appeal the verdict and sentence... it is unjust for my client. He did not know or commit many of these crimes," Son Arun, a lawyer for Nuon Chea, told reporters.

Nil Nonn however told the court that "given the gravity of the crimes" both men would remain in detention.

Amnesty International hailed the trial evrdicts as a crucial step towards justice.

"This long-awaited ruling is an important step towards justice for the victims of the Khmer Rouge period and highlights the importance of addressing impunity," the group's deputy Asia-Pacific director Rupert Abbott said.

However, he said earlier refusal of senior Cambodian government officials to give evidence, as well as allegations of political interference in other cases, were "troubling and raises concerns around the fairness of the proceedings and respect for victims’ right to hear the full truth regarding the alleged crimes".

Al Jazeera's Rob McBride, reporting from the court, said there were emotional scenes from a crowd of survivors of the "Killing Fields" era outside the courthouse when the judgement was announced.

A few dozen survivors, many travelling from far-flung rural provinces, had joined about 900 Cambodians at the Phnom Penh-based court.

"I have been waiting for justice since 1979 ... This is a very important day for me," said Yann Sok, 76, who lost 20 relatives including two of his sons under the regime.