President Mahmoud Ahmadinejad of Iran on Monday used the platform of a United Nations conference in Geneva on combating racism to disparage Israel as a “cruel and repressive racist regime,” prompting delegates from European nations to desert the hall and earning a rare harsh rebuke from Secretary General Ban Ki-moon.

As Mr. Ahmadinejad began to speak, two protesters wearing rainbow-hued clown wigs  their statement on the tenor of the proceedings  pelted him with red foam noses. Hustled out the door by security agents, they were soon followed by lines of stony-faced diplomats from the 23 European nations attending the conference. They walked out to the sound of some other delegates applauding Mr. Ahmadinejad.

The United States and more than a half-dozen other nations had already boycotted the gathering out of concern that it would focus on maligning Israel rather than on the global problems of discrimination, replaying the disputes that marked the first United Nations conference on combating racism in Durban, South Africa, in 2001.

Years of negotiations intended to avoid just such a scenario failed, underscoring the uneasy gap that exists between the rest of the world and the West when it comes to certain issues, like whether Israel’s treatment of the Palestinians under occupation belongs at a forum on discrimination and xenophobia.