Dozens of Western diplomats walked out of a U.N. conference and a pair of rainbow-wigged protesters threw clown noses at Iran''s president Monday when the hard-line leader called Israel the "most cruel and repressive racist regime."

The United States decried the remarks by Mahmoud Ahmadinejad as hateful — reinjecting tension into a relationship that had been warming after President Barack Obama sought to engage Iran in talks on its nuclear program and other issues.

Ahmadinejad — the first government official to take the floor at the weeklong event in Geneva — delivered a rambling, half-hour speech that was by turns conciliatory and inflammatory. At one point he appealed for global unity in the fight against racism and then said the United States and Europe helped establish Israel after World War II at the expense of Palestinians.

"They resorted to military aggression to make an entire nation homeless under the pretext of Jewish suffering," he said.

Jewish groups had lobbied heavily for a boycott of the conference, warning it could descend into anti-Semitism or other anti-Israel rhetoric, which marred the last such conference eight years ago in South Africa.

The meeting turned chaotic almost from the start when the two wigged protesters tossed the red clown noses at Ahmadinejad as he began his speech with a Muslim prayer. A Jewish student group from France said it had been trying to convey "the masquerade that this conference represents."