The US led an enraged Western walkout after Mr Ahmadinejad's latest incendiary remarks, which also attacked Israel and accused the West of monopolising nuclear power - at a time when his country faces nuclear sanctions.
Two US officials who had been watching the speech led the protest over the September 11 comments.
The British and other European Union delegations quickly followed after Mr Ahmadinejad said most Americans believed their government was involved in the 2001 attacks. Canada boycotted the speech even before it started.
The Iranian president said there was a theory that “some segments within the US government orchestrated the attack to reverse the declining American economy and its grips on the Middle East in order also to save the Zionist regime”.
“The majority of the American people as well as other nations and politicians agree with this view,” he declared to the astonished chamber.
About 3000 people died in the attacks by al-Qa'ida suicide attackers who hijacked jets and flew them into the World Trade Centre in New York, the Pentagon in Washington and a field in Pennsylvania.
The US mission at the UN furiously slammed the Iranian leader in a statement released even before Mr Ahmadinejad had finished speaking.
“Rather than representing the aspirations and goodwill of the Iranian people, Mr Ahmadinejad has yet again chosen to spout vile conspiracy theories and anti-Semitic slurs that are as abhorrent and delusional as they are predictable,” said Mark Kornblau, spokesman for the US mission. Read more