Ahmadinejad smiled at first but appeared increasingly agitated, decrying the "insults" and "unfriendly treatment." Columbia president Lee Bollinger and audience members took him to task over Iran's human-rights record and foreign policy, as well as Ahmadinejad's statements denying the Holocaust and calling for the disappearance of Israel.
"Mr. President, you exhibit all the signs of a petty and cruel dictator," Bollinger said, to loud applause.
He said Ahmadinejad's denial of the Holocaust might fool the illiterate and ignorant.
"When you come to a place like this it makes you simply ridiculous," Bollinger said. "The truth is that the Holocaust is the most documented event in human history."
Ahmadinejad rose, also to applause. After a religious invocation, he said Bollinger's opening was "an insult to information and the knowledge of the audience here."
"There were many insults and claims that were incorrect, regretfully," Ahmadinejad said, accusing Bollinger of falling under the influence of the hostile U.S. press and politicians. "I should not begin by being affected by this unfriendly treatment."
During a question and answer session, Ahmadinejad appeared tense and unsmiling, in contrast to more relaxed interviews and appearances earlier in the day.
In response to one audience member, Ahmadinejad denied he was questioning the existence of the Holocaust: "Granted this happened, what does it have to do with the Palestinian people?"
But then he said he was defending the rights of European scholars, an apparent reference to a small number who have been prosecuted under national laws for denying or minimizing the Holocaust.
"There's nothing known as absolute," he said.
He reiterated his desire to visit ground zero to express sympathy with the victims of the Sept. 11 attacks, but then appeared to question whether al-Qaida was responsible.
"Why did this happen? What caused it? What conditions led to it?" he said. "Who truly was involved? Who was really involved and put it all together?"
Asked about executions of homosexuals in Iran, Ahmadinejad said the judiciary system executed violent criminals and high-level drug dealers, comparing them to microbes eliminated through medical treatment. Pressed specifically about punishment of homosexuals, he said: "In Iran we don't have homosexuals like in your country."
With the audience laughing derisively, he continued: "In Iran we do not have this phenomenon. I don't know who's told you that we have this."
Bollinger was strongly criticized for inviting Ahmadinejad to Columbia, and had promised tough questions in his introduction to Ahmadinejad's talk. But the strident and personal nature of his attack on the president of Iran was startling.
"You are either brazenly provocative or astonishingly uneducated," Bollinger told Ahmadinejad about the leader's Holocaust denial.
During his prepared remarks, the Iranian president did not address Bollinger's accusations directly.
Suzanne Maloney, a foreign policy fellow at the Brookings Institution, said Ahmadinejad's softer tone on Israel in this speech may reflect backlash in his own country.
"There's been widespread commentary in Iran, even on the far-right, that Ahmadinejad's position on Israel has hurt the country's diplomatic relations," said Maloney. "The fact that he was frankly unwilling to go as far as he has in the past suggests there may have been some consequences for him at home."
President Bush said Ahmadinejad's appearance at Columbia "speaks volumes about really the greatness of America."
He told Fox News Channel that if Bollinger considers Ahmadinejad's visit an educational experience for Columbia students, "I guess it's OK with me."
Other American officials were less sympathetic.
On Capitol Hill, conservatives said Columbia should not have invited Ahmadinejad to speak. Senate Republican Leader Mitch McConnell, R-Ky., said "there is a world of difference between not preventing Ahmadinejad from speaking and handing a megalomaniac a megaphone and a stage to use it."
Sen. Joseph Lieberman, I-Conn., said he thought Columbia's invitation to Ahmadinejad was a mistake "because he comes literally with blood on his hands."
Thousands of people jammed two blocks of 47th Street across from the United Nations to protest Ahmadinejad's visit to New York. Organizers claimed a turnout of tens of thousands. Police did not immediately have a crowd estimate.
The speakers, most of them politicians and officials from Jewish organizations, proclaimed their support for Israel and criticized the Iranian leader for his remarks questioning the Holocaust.
"We're here today to send a message that there is never a reason to give a hatemonger an open stage," New York City Council Speaker Christine Quinn said.
Protesters also assembled at Columbia. Dozens stood near the lecture hall where Ahmadinejad was scheduled to speak, linking arms and singing traditional Jewish folk songs about peace and brotherhood, while nearby a two-person band played "You Are My Sunshine."
Signs in the crowd displayed a range of messages, including one that read "We refuse to choose between Islamic fundamentalism and American imperialism."