Rwanda girls light each other's candles during a night vigil for the commemoration of the 20th anniversary of the genocide, in the Rwandan capital Kigali April 7, 2014. An estimated 800,000 people were killed in 100 days during the genocide. [Photo/Agencies]

KIGALI - The United Nations chief told a packed stadium of sombre and weeping Rwandans on Monday the world would "never again" let genocide tear their nation apart, at a ceremony marking 20 years since 800,000 people were butchered.

A host of leaders and donors attended the commemoration, but France - an ally of the Rwandan government that ruled before the genocide - did not take part after rebel-turned-president, Paul Kagame, renewed charges of Paris' "direct role" in the killings.

France has acknowledged mistakes in its dealings with Rwanda. But it has repeatedly dismissed accusations it trained militias to take part in the massacres and Kagame's comments triggered fresh outrage in Paris on Monday.

Some in the crowd in Kigali were overcome with emotion on hearing a survivor's account and stewards had to lead them out of the stadium. Many Rwandans lost entire families to killers armed with guns, grenades, machetes and cans of petrol.

A minute's silence was punctuated by screams of dozens of survivors.

"We must not be left to utter the words 'never again', again and again," UN Secretary-General Ban Ki-moon told the crowd.

"Many United Nations personnel and others showed remarkable bravery. But we could have done much more. We should have done much more," he said, adding there were new challenges in the region.

Conflicts rumble on in South Sudan and the Central African Republic, while the eastern Democratic Republic of Congo next door remains in turmoil.