Iranian President Mahmoud Ahmadinejad triggered a mass exodus from the U.N. General Assembly’s chamber Thursday with a combative speech that blasted the United States and other Western powers and questioned whether Islamist terrorists were behind the Sept. 11, 2001, attacks.

The Iranian leader, known for his bomb-throwing rhetoric, used his allotted 15 minutes before the world body to blame the West for a list of ills throughout history, from slavery to the two world wars and the global economic crisis.

He also criticized the Obama administration for killing Osama bin Laden, suggesting that the al-Qaeda leader could have been the star witness at a trial that would reveal the true culprits behind the attacks on New York and Washington.

“Instead of assigning a fact-finding team, they killed the main perpetrator and threw his body into the sea,” Ahmadinejad said. Meanwhile, those who raised questions about Sept. 11 or the Nazi Holocaust were “threatened with sanctions and military action,” he said.

His words sent diplomats streaming for the exits, starting with the U.S. delegation and followed by dozens of Europeans and others. More than a third of the General Assembly seats were empty by the time Ahmadinejad finished speaking, to polite applause.

The Iranian president had recently made conciliatory gestures to the West, including his support for a decision to free Americans Shane Bauer and Josh Fattal, who were arrested in 2009 after straying into Iranian territory during a hike along the Iraqi border. But there were no olive branches in evidence during his sharply worded speech, which was accompanied by finger-wagging and dramatic hand gestures.

“Do these arrogant powers really have the competence and ability to run or govern the world?” he asked, referring to the United States and the former colonial powers of Europe. In an apparent reference to the Western-led military intervention in Libya, he added: “Can the flower of democracy blossom from NATO’s missiles, bombs and guns?”

Positioning himself as spokesman for developing and non-aligned countries, Ahmadinejad called for scrapping the “prevailing world order” — including the U.N. Security Council as currently structured — in favor of a more evenly balanced system.

“There is no other way than the shared and collective management of the world to put an end to the present disorders, tyranny and discriminations worldwide,” he said.