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.

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.