Rwanda on Monday commemorated the 20th anniversary of the genocide there with a fusion of tears, recrimination and regret at the killing of more than 800,000 people in 100 days that shocked the world, redrew regional battle lines and continues to shape international debate over how nations should respond to mass atrocities.

In the run-up to the commemorations, President Paul Kagame rekindled an old dispute with France — an influential player in Francophone Africa, even through Rwanda had once been a Belgian colony — accusing France of helping the killers escape and of playing a “direct role.” He seemed to allude to those allegations, frequently denied by France, as he spoke to the gathering on Monday.

“Behind the words ‘never again’ there is a story whose truth must be told in full,” he said.

“No country is powerful enough, even when they think they are, to change the facts,” Mr. Kagame said in a seeming reference to France, Reuters reported. “Facts are stubborn.”

In response to Mr. Kagame’s accusations, France, which had trained the Hutu-dominated military under President Habyarimana, canceled plans for its justice minister, Christiane Taubira, to attend Monday’s ceremony. Overnight, the French ambassador in Kigali, Michel Flesch, said he was told by an official that he was no longer accredited to attend the memorial, news reports said.