UNITED NATIONS — President Mahmoud Ahmadinejad of Iran made a series of incendiary remarks in his speech to the United Nations General Assembly on Thursday, notably the claim that the United States orchestrated the Sept. 11 attacks to rescue its declining economy, to reassert its weakening grip on the Middle East and to save Israel.

Those comments prompted at least 33 delegations to walk out, including the United States, Canada, Australia, New Zealand, Costa Rica, all 27 members of the European Union and the union’s representative, diplomats said.

The annual General Assembly started formally on Thursday, with scores of presidents, kings and ministers expected to address the gathering over the coming week. The speeches often fail to break new ground or lack electricity, so the occasional theatrics inevitably attract considerable attention.

Mr. Ahmadinejad rarely disappoints on that scale, although he seemed to go out of his way to sabotage any comments he made previously this week about Iran’s readiness for dialogue with the United States. The theme of his often flowery speech was that the capitalist world order was collapsing and he cited three examples: the attacks of Sept. 11, 2001, the Israeli occupation of Palestinian lands, and the pressure on Iran over its nuclear program.