Tens of thousands of Cambodians have packed into a stadium in Phnom Penh to mark the 30th anniversary of the fall of the murderous Khmer Rouge.

People hold up a banner showing a dove, the symbol of peace, as Cambodia's Prime Minister Hun Sen (R), President of the ruling Cambodian People's Party (CPP) Chea Sim (C) and senior party member Heng Samrin (L) pass by in a car at the National Olympic Stadium in Phnom Penh on January 7, 2009, to mark the 30th anniversary of the toppling of Pol Pot's Khmer Rouge regime. Some 1.7 million people are believed to have died in the "Killing Fields" of the ultra-Maoist guerrillas, whose four year reign of terror was brought to an end in 1979 by invading troops from neighboring Vietnam. (Reuters/Chor Sokunthea/Cambodia) Senate President Chea Sim lauded "those who sacrificed their lives to save us from genocide", when Vietnamese-led forces ousted the regime in 1979.

Up to two million people died over the four years of Khmer Rouge rule.

But none of its surviving leaders have yet faced justice, triggering criticism of foot-dragging by the government.

A UN-backed war crimes trial of five henchmen of late leader Pol Pot is expected to begin in the next few months.

"The spirits of my relatives will not be calm without prosecuting those killers," Thay Srey Khon - who lost eight relatives under the regime - told Reuters news agency.

'Dark chapter'

More than 40,000 people filled Phnom Penh's Olympic stadium for the event, organised by the ruling Cambodian People's Party (CPP).

Some held up coloured tiles to spell out slogans which stretched across the crowd, says the BBC's Guy De Launey from the scene.

Their reward for sitting in the baking sun was to watch a variety of entertainers parading across the football field - a marching band, colourful floats, and traditional dancers and musicians - our correspondent adds.

Chea Sim said the anniversary marked the end of "the dark chapter of Cambodian history" - and thanked neighbouring Vietnam for "saving the country from genocide".

But opposition party leaders were conspicuous by their absence from the celebrations, and correspondents say this interpretation of the country's recent past is not uncontested.

A significant minority of Cambodians mourn the anniversary as the beginning of a decade-long occupation by Vietnam. Prime Minister Hun Sen has dismissed the critics, saying that people who oppose the celebrations are "either Pol Pot or animals."

The CPP is itself a descendant of the puppet government installed by Vietnam after the fall of the Khmer Rouge.

Justice elusive

In his speech Chea Sim made no mention of the tribunal which is expected to begin trying five remaining Khmer Rouge leaders in March.

The first to go on the stand is expected to be Kaing Guek Eav - better known as Comrade Duch, head of the notorious Tuol Sleng torture centre.

Top leaders in the current ruling Cambodian People's Party served in the ranks of the Khmer Rouge before defecting, and the government has been accused of attempting to delay the start of tribunal proceedings, and of trying to curtail the scope of tribunal investigations.

"After 30 years, no-one has been tried, convicted or sentenced for the crimes of one of the bloodiest regimes of the 20th Century," the New York-based Human Rights Watch said in a statement on Monday.

"This is no accident."

It accused Prime Minister Hun Sen of "doing his best to thwart justice", and said in addition both the US and China and other nations had "blocked efforts at accountability".

Both, it said, had continued to support the Khmer Rouge for years after its downfall, each for its own reasons.