* France absent after row over Kagame remarks
* Rebel-turned-statesman Kagame led rebuilding of nation
* Genocide in 1994 killed 800,000 in 100 days (Adds British foreign minister's comments, background)
By Jenny Clover
KIGALI, April 7 (Reuters) - 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," U.N. 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.
Rwanda has long complained that other nations stood idle when the massacres erupted in April 1994, mostly of people from the Tutsi minority but also moderates among the Hutu majority.
"Behind the words 'never again' there is a story whose truth must be told in full," the president told attendees, who watched performers dressed in grey symbolically re-enacting some of the horrors.
Rwandans carried out the genocide, "but the history and root causes go beyond this beautiful country", he said.
"No country is powerful enough, even when they think they are, to change the facts," he said in an apparent swipe at France. In a speech in English and the Kinyarwanda language, he added in French: "Facts are stubborn," drawing applause.
Kagame, a Tutsi who led an army into Kigali in 1994 to halt the genocide, has in the past accused France of training and arming Hutu extremists. Recently he had seemed to have dropped the issue and ties had been slowly improving.
But in an interview in a weekly journal published this month he said France and former colonial power Belgium had a "direct role" in the genocide. In response, France said it would not send a ministerial delegation.
On Monday, France's foreign minister in 1994, Alain Juppe, demanded President Francois Hollande defend France's honour against the accusations. "Rwanda's regime has made a habit of repeatedly falsifying history," Juppe told reporters.
Hollande avoided reference to the row, saying in a statement: "On this day of commemoration, France stands by all Rwandans to honour the memory of all the victims of the genocide."
Kagame has dropped military fatigues for sharp suits and has been praised for attracting investors, building an efficient health system and reducing poverty, but is also criticised for an authoritarian style.
Western nations cut some aid in 2012 after his government was accused of backing rebels in neighbouring Congo. South Africa has also accused Kigali of sending hit squads to kill exiled opponents on South African soil.
Rwanda vigorously denies both charges.
Many Rwandans, who have seen dramatic changes with new roads and other benefits, say Kagame has helped unite the nation.
"Before I would feel ashamed or afraid of saying anything at gatherings but now we feel that we are all Rwandan with no picture of ethnicity," Samuel Munyarugerero, a 45-year-old Hutu and former soldier in the army before the genocide.
"Yes, Rwandans are reconciled."
Some welcome the gains but worry about the methods.
"The politics of Rwanda is coercive," said 33-year-old Hutu agronomist from Kigali, who like other critics asked not to be named for fear of repercussions. "If the government could mobilize people with no coercion, Rwanda would be much better."
Kagame rejects the charge of restricting political rights.
Speaking in Kigali, British Foreign Secretary William Hague on Monday called for more freedoms in the country.
"We have to acknowledge the very special circumstances of the last 20 years, the leadership and the direction that was necessary to pull a country together," he said.
"But of course we would argue, yes, political space needs to be opened up over the coming years." (Additional reporting by Clement Uwiringiyimana in Kigali and John Irish in Paris; Writing by Edmund Blair; Editing by Andrew Roche)
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