Protesters demonstrate outside the United Nations headquarters against Iranian President Mahmoud Ahmadinejad as he addresses the General Assembly. (Eric Thayer/Reuters)

American delegates to the UN General Assembly stood up and walked out on an address by Iranian President Mahmoud Ahmadinejad on Thursday after he suggested the United States planned the Sept. 11 attacks to protect its own economy and support Israel.

Canada boycotted the speech, with Foreign Affairs Minister Lawrence Cannon calling the Iranian president's comments about 9/11, the U.S. and Israel "unacceptable" and an affront to the spirit of the UN.

In his address, Ahmadinejad accused some segments within the U.S. government of orchestrating the attack "to reverse the declining American economy and its grips on the Middle East in order to save the Zionist regime."

"The majority of the American people, as well as most nations and politicians around the world, agree with this view."

Iranian President Mahmoud Ahmadinejad waves as he concludes his UN speech Thursday. (Shannon Stapleton/Reuters)

This was the second of three viewpoints that Ahmadinejad claimed exist regarding the 2001 attacks, which killed 2,974 people in New York City, Washington, D.C., and Shanksville, Pa.

The first, he asserted, was that "a very powerful and complex terrorist group" planned them; the third, that they were carried out "by a terrorist group, but that the American government supported and took advantage of this situation."

He said the U.S. used the 9/11 attacks as a pretext to invade Afghanistan and Iraq that led to the killing of hundreds of thousands of people, saying the U.S. should have "designed a logical plan" to punish the perpetrators while not shedding so much blood.

Delegates from other countries also walked out during the speech. If Ahmadinejad noticed, he did not show it, as he railed against American intervention in Iraq and Afghanistan, as well as against Israel's "occupation of the Palestinian territories," and negative attitudes toward nuclear energy, which he called "clean and cheap and a heavenly gift."

He then announced that Iran would host a conference to study terrorism and ways to confront it, and invited officials and researchers "from all countries" to attend.

The Canadian delegation had boycotted the Iranian president during a speech on Tuesday when he addressed the UN's Millennium Development Goals summit.

"Ahmadinejad's comments on the 9/11 terrorist attacks, on the United States, and the repeated unjust condemnation of Israel are unacceptable," Cannon said in his statement.

"Iran's 20-year history of covering up its nuclear activities requires that the authorities take steps to address a serious confidence deficit. Its non-compliance, coupled with unacceptable statements Iran has made against other nations, is a destabilizing threat to the region, and to the world."

Protest outside UN

Outside the UN, about 2,000 Iranian-Americans gathered on Dag Hammarskjold Plaza to protest Ahmadinejad's appearance.

They carried red, white and green Iranian flags, and red, white and green umbrellas to ward off the hot autumn sun. Some held purple balloons and many wore purple, which one of the organizers, Ali Safavi, called "the colour of democracy."

There was also a large papier-mâché representation of Ahmadinejad's head with a nuclear missile strapped to the back.

Former New York mayor Rudy Giuliani turned out to support the demonstrators, telling them they had the support "of all democratic people."

"Your goals are our goals," Giuliani said.

"They are the goals of all democratic people. You want to see freedom of religion … of the press," he went on, to loud cheers. "We must confront tyrants, oppressors, bullies and terrorists as early as possible."

With files from The Associated Press