"Ahmadinejad’s speech contradicted the spirit and purpose of the conference, which is to defeat the scourge of racism," said Human Rights Watch spokeswoman Juliette de Rivero.
"The best response to Ahmadinejad’s inflammatory rhetoric is to stay in Geneva and rebut it."
Ahmadinejad's speech at the conference criticised the creation of a "totally racist government in occupied Palestine" in 1948, calling the Israeli administration "the most cruel and repressive racist regime."
His remarks prompted 23 European Union delegations to walk out in protest and the Czech Republic became the 10th nation to announce it was boycotting the event.
De Rivero called on those boycotting to reconsider.
"Despite this ugly speech, governments can still rescue the conference and ensure that the world agrees to a strong mandate for the UN to tackle racism. Racism does real damage to real people and it shouldn’t be manipulated for political ends," she said.
Britain and France, who both denounced the speech, said they would remain at the conference.
The United States and Israel had already led nine nations in boycotting the event before it started.
Concerned it would denegerate into a forum for anti-Semitism, Poland, Germany, Italy, the Netherlands, Canada, Australia and New Zealand also said they would stay away from the event.
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