The ceremony comes at the start of a week of events
Rwanda has held a candlelit vigil at the start of a week of national mourning to mark 15 years since the genocide which killed 800,000 people.
Ceremonies were held in the capital Kigali, and in Nyanza, where more than 5,000 people were slaughtered.
At a stadium in Kigali, thousands of candles spelt out the word "hope" in three languages.
UN Secretary General Ban Ki-moon has said that preventing future genocide was "a collective responsibility".
"Only by meeting this challenge can we match the resolve of the survivors and truly honour the memory of those who died in Rwanda 15 years ago," he said.
"The resounding voices of survivors touch us in ways that no other words could. Yet the silence of the more 800,000 innocent victims still haunts our collective conscience."
Karen Allen at the genocide vigil
US President Barack Obama said the genocide was "so enormous, so daunting, that it runs the risk of becoming a statistic".
He said it was important to remember that each person who died had "their own story, their own family, and their own dreams" and that remembering such events deepened the commitment to prevent "future atrocities".
Mr Obama also praised the "courageous" survivors who he said had "demonstrated remarkable strength and generosity in forgiving those who committed these heinous acts".
"These individuals inspire us daily by working to restore trust and rebuild hope in Rwanda," he said.
RWANDA'S 1994 GENOCIDE
Rwandan Hutu President Habyarimana's plane shot down
An estimated 800,000 Tutsis and moderate Hutus killed
Tutsi-led rebel group RPF captures Rwanda's capital Kigali
Two million Hutus flee to Zaire, now DR Congo
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