The US and other Western delegations walked out in protest during a speech by Iran's president at the UN, in which he said most people believed the US government was behind the 9/11 attacks.
Mahmoud Ahmadinejad was speaking at the UN General Assembly in New York.
The US was joined in its walk-out by representatives from all the EU countries, Canada, Australia, New Zealand and Costa Rica.
The US described his remarks as "abhorrent and delusional".
But Mr Ahmadinejad appeared undaunted by the protest, going on to attack Zionism and Israel.
The Iranian president's speech was part political diatribe, part sermon, says BBC diplomatic correspondent Jonathan Marcus - a wide-ranging presentation of his own particular world view.
The Iranian president said he would host a conference on terrorism next year and that 2011 should be the year of nuclear disarmament.
Repeating Iran's denial that it was seeking the capacity to build nuclear weapons, Mr Ahmadinejad said some members of the UN Security Council had "equated nuclear energy with nuclear bombs".
Mr Ahmadinejad had started his speech by outlining what he called the failure of the existing world order and capitalism, saying the world should be run by virtuous people like the Prophets.
Although he said that Iran was ready to have a serious discussion with US statesmen, our correspondent says that on the evidence of this speech there would be little for them to talk about.
Addressing the General Assembly, Mr Ahmadinejad said it was mostly US government officials and statesmen who believed that al-Qaeda militants carried out the 2001 attacks on the US.
He said another theory was "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 to save the Zionist regime". Mr Ahmadinejad usually refers to Israel as the "Zionist regime".
"The majority of the American people as well as most nations and politicians around the world agree with this view", Mr Ahmadinejad said.
In response to the speech, Mark Kornblau, spokesman for the US mission at the UN, told AFP news agency: "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."
Iran has already endured four rounds of increasingly punitive economic sanctions over the nuclear dispute.
Foreign ministers from countries including the US, UK, China, France, Germany and Russia discussed the issue at a meeting on Wednesday, and it is also likely to be raised on the fringes of the General Assembly.
Dozens of fringe meetings take place and correspondents say they constitute the most important business of the event.
Peace-keeping in Somalia, the possible break-up of Sudan, the conflict in Yemen, climate change and UN reform are all set to feature in these smaller meetings.
Speaking shortly after UN Secretary General Ban Ki-moon launched the diplomatic marathon on Thursday, US President Barack Obama urged the audience to support direct Israeli-Palestinian peace negotiations which started on 2 September.
He said those longing for an independent Palestine must not try to tear down Israel, and called on Israel to extend a moratorium on building new settlements in the West Bank and East Jerusalem.