Ahmadinejad questioned the Holocaust and said the Western nations had used it as an excuse for unwavering support for Israel and for the oppression of the Palestinian people.
The US said Ahmadinejad again wasted a chance to address at an international forum the concerns of his people and instead used his speech at the UN General Assembly for "despicable conspiracy theories''.
A US diplomat who was in the assembly hall to monitor the speech left halfway through, while the 27 European Union nations then followed in a coordinated protest move.
In what has become a sort of ritual at the annual UN General Assembly, US diplomats led a walkout during Ahmadinejad's speech here today in which the controversial Iranian leader referred to the September 11 attacks as "mysterious".
He even questioned America's raid that killed al-Qaeda leader Osama bin Laden.
"Would it not have been reasonable to bring to justice and openly bring to trial the main perpetrator of the incident in order to identify the elements behind the safe space provided for the invading aircraft to attack the twin World Trade Center towers?"
"Instead of assigning a fact-finding team, they killed the main perpetrator and threw his body into the sea," he said.