Rwanda's president said the international community "failed all of us", as he marked 30 years since the 1994 genocide that killed around 800,000 people.
President Paul Kagame addressed dignitaries and world leaders who had gathered in Rwanda's capital, Kigali, to commemorate the bloodshed.
"Rwanda was completely humbled by the magnitude of our loss," he said.
"And the lessons we learned are engraved in blood."
On this day in 1994, extremists from the Hutu ethnic group launched a 100-day killing spree, in which members of the Tutsi minority and Hutu moderates were slaughtered.
The mainly Tutsi forces who took power following the genocide were alleged to have killed thousands of Hutu people in Rwanda in retaliation.
On Sunday, Mr Kagame and a group of dignitaries placed wreathes on mass graves at the Kigali Genocide Memorial - where more than 250,000 victims are believed to be buried. The president also lit a remembrance flame.
In a speech later, Mr Kagame thanked fellow African countries including Uganda, Ethiopia and Tanzania for their assistance in ending the genocide.
"Many of the countries representing here also sent their sons and daughters to serve as peacekeepers in Rwanda," he said.
"Those soldiers did not fail Rwanda. It was the international community which failed all of us. Whether from contempt or cowardice.
