Top diplomats observed a minute of silence Wednesday after lighting five white candles in memory of more than 500,000 people killed in the Rwandan genocide 18 years ago. They vowed to pursue justice for the victims and survivors by apprehending fugitive Rwandan killers.

U.S. Ambassador Susan Rice recalled that the "United Nations was established in the shadow of a genocide," the Nazi slaughter of six million Jews during World War II.

"It suffered an enormous blow to its credibility and effectiveness in the face of another genocide, the one we're gathered to commemorate today," she said, a reference to the U.N.'s failure to intervene and halt the Rwanda genocide.

Rice reminded the gathered diplomats that the first inkling that a genocide had begun in Rwanda came when the U.N. peacekeeping mission chief, Canadian Maj.-Gen. Romeo Dallaire, was called to the morgue room of a Kigali hospital.

"In a dark room, the beam of his flashlight revealed what was left of 10 Belgian peacekeepers, mutilated beyond recognition. In the same hospital, 100 times that number of innocent Rwandans lay dead. And this was just the beginning."

Rwanda's genocide began hours after a plane carrying President Juvenal Habyarimana was mysteriously shot down as it approached the capital, Kigali, on April 6, 1994.

The 100-day slaughter, in which more than 500,000 minority Tutsis and moderate members of the Hutu majority were killed by Hutu extremists, ended after Tutsi-led rebels ousted the extremist Hutu government that orchestrated the killings.

Rwanda's U.N. Ambassador Eugene-Richard Gasana said that "Genocide is a meticulously thought-out atrocity, harbored in the minds and hearts" of the perpetrators, and rehearsed on various occasions: in 1959, when thousands of Tutsis were killed and driven from their homes, again in the 1960s, and in 1973, when young Tutsis were killed in their schools.

It all led to the "1994 apocalypse," he said.

Gasana warned against genocide denial and trivialization, and appealed for the arrest and trial of Rwandan killers who have found shelter in other nations, where they have become politically active and propagandized against the history of Rwanda's slaughter.

In a taped message, U.N. Secretary-General Ban Ki-moon, who was en route to Europe, said: "To those who persist in suppressing their fellow citizens, who cry out for dignity and freedom, we send a clear message: justice must be done."

"Our eyes will never be closed again," promised the General Assembly's president, Nassir Abdulaziz al-Nasser.