Iranian President Mahmoud Ahmadinejad defended Holocaust revisionists and raised questions about who carried out the September 11 attacks in a tense showdown at Columbia University on Monday, where the school's head introduced the hard-line leader by calling him a "petty and cruel dictator."

Ahmadinejad portrayed himself as an intellectual and argued that his administration respected reason and science. But the former engineering professor, appearing shaken and irate over what he called "insults" from his host, soon found himself drawn into the type of rhetoric that has alienated American audiences in the past.

He provoked derisive laughter by responding to a question about Iran's execution of homosexuals by saying: "In Iran we don't have homosexuals like in your country ... I don't know who's told you that we have this."