"Women and girls were gang-raped, tortured and maimed for life, if not murdered. The victims were forced to kill their kin or dig their own graves before they were buried alive," President Paul Kagame told the 28,000 people gathered at a stadium in the capital Kigali.
"We will see each other again in heaven," a choir sang at the memorial site where a crowd of barefoot Rwandans in tattered clothes watched from a hilltop as African presidents arrived in gleaming vehicles.
In Geneva, UN chief Kofi Annan said the risk of genocide remained frighteningly real in parts of the world, explaining that Rwanda-style ethnic massacres may be in the making in Sudan and international military force could be needed to stop it.
The UN Secretary-General 's initiative on Sudan stood in stark contrast to the low-key approach adopted by the international community on Rwanda in the months leading up the genocide, a failure seen by many Rwandans as a disastrous abdication of responsibility.
Mr Kagame singled out France for particular scorn, reiterating an accusation it had helped train the killers knowing they would commit a genocide a charge France strongly denies.
"As for the French, their role in what happened in Rwanda is self-evident," Mr Kagame said. "They knowingly trained and armed government soldiers and militia who were going to commit genocide, and they knew they would commit a genocide."
Mr Kagame lit an eternal flame at the main memorial site as workers buried
15 coffins in a mass grave nearby and later led
Rwandans in observing two minutes' silence for the victims.
The silence at the stadium was eventually broken by a chorus of women clad in the Rwandan mourning colour of purple.
Bereaved survivors later sobbed in shock at hearing witnesses tell the stadium audience about gruesome killings they had seen.
For many ordinary Rwandans, most of whom scratch a living as peasant farmers in one of the world's poorest countries, the trauma is far from healed.
Many women were infected with AIDS during mass rapes, and thousands of children were orphaned.
"It will take eternity for the detestable and guilty indifference of the international community to be forgotten," said Louis Michel, foreign minister of the former colonial power Belgium, which lost 10 peacekeeping troops to Hutu killers on April 7, prompting Brussels to withdraw its other soldiers.
April 7 has been designated by the United Nations as an International Day of Reflection for Rwanda, and the African country had asked other nations also to hold memorial silences.
But apart from ceremonies due to take place at UN offices in major UN centres such as Nairobi and Geneva and at Rwandan embassies, there was no sign the gesture was widely observed.