Rwanda is marking 20 years since the start of the 1994 genocide that left 800,000 people dead.
At a ceremony Monday in Kigali, President Paul Kagame and U.N. Secretary-General Ban Ki-moon joined in lighting a national flame of mourning that will burn for 100 days.
That is equal to the amount of time ethnic Hutu militias rampaged across the country, killing Tutsis and moderate Hutus after the death of Rwanda's president in a plane crash.
The ceremony brought in heads of state and dignitaries from the United States, Britain and East African nations.
Mr. Ban said Sunday he would reaffirm the international community's commitment that such atrocities "should never happen in human history."
He also praised Mr. Kagame's leadership, saying the president has "made shining examples" of socioeconomic success, democracy and human rights.
U.S. President Barack Obama said in a statement marking the anniversary that the genocide was a "deliberate and systematic effort" by humans to destroy other humans. He said when faced with hatred and cruelty, people must remember their shared humanity, choose compassion and "never be indifferent."
Mr. Obama also said we must remember the world's failure to respond to the situation more quickly.
In the early 1990s, the United Nations had several thousand peacekeepers in Rwanda who have been criticized for their inability or unwillingness to stop the genocide, despite warnings.
In 1999, an independent inquiry ordered by then-U.N. chief Kofi Annan said the inability to prevent or stop the genocide "was a failure by the United Nations system as a whole." The report said member states did not have the will to act with assertiveness, and that there were "serious mistakes" made with the resources that were available to the U.N. mission in Rwanda.
At an international forum on genocide in Kigali Sunday, Rwanda's government reiterated remarks that France played a "direct role" in the genocide.
Foreign Minister Louise Mushikiwabo said France has "wronged" Rwanda by being partially responsible for the genocide. She said relations between France and Rwanda cannot be repaired if Rwanda has to accept the French version of events.
She echoed Mr. Kagame's remarks that France and Belgium played what he called a "direct role" in the political preparation for the genocide. The president's remarks led France to cancel plans to attend the ceremonies this week.
France has denied any responsibility for the genocide and says the accusations go against the reconciliation process between the two countries.
In Rome, Pope Francis - the head of the Roman Catholic Church - said Sunday he wanted to express his "paternal closeness to the Rwandan people" and encouraged them to continue the process of reconciliation "with determination and hope."