Mr Ahmadinejad also insisted in a forum at Columbia University Iran's nuclear program was purely peaceful. Challenged over his past comments that Israel should be wiped off the map and questioning the Holocaust, he said his concern was why the Palestinians were suffering.
Mr Ahmadinejad received a caustic welcome at the university, which had come under fire from critics who said it should not give a platform to a Holocaust denier.
Security was tight at the hall where he addressed around 700 people, 80 per cent of them students -- dozens of whom were wearing T-shirts saying "Stop Ahmadinejad's Evil."
Introducing him, Columbia University President Lee Bollinger said that Mr Ahmadinejad behaved as a "petty and cruel dictator" and that his Holocaust denials suggested he was either "brazenly provocative or astonishingly uneducated."
Mr Bollinger asked a string of tough questions, most of which Mr Ahmadinejad ignored in a 30-minute speech which dwelt at length on science as a gift from God and the importance of using knowledge and learning purely and in a pious way.
Answering questions about his views on the Holocaust, Mr Ahmadinejad said there were no absolutes in academia, and "a different perspective" was necessary given the impact on the Middle East of those events.
"I'm not saying that it didn't happen at all," he said. "I said, granted this happened, what does it have to do with the Palestinian people."
Mr Ahmadinejad also rejected criticism of human rights in his country, notably persecution of homosexuals: "In Iran we do don't have homosexuals like in your country," he said, sparking loud laughter from the audience. "In Iran we do not have this phenomenon. I do not know who has told you we have it."
Outside, several hundred protesters objected to his being allowed to speak at the university.
Earlier, in a video conference with reporters in Washington, Mr Ahmadinejad accused Israel of occupation and racism."
We do not recognize that regime (Israel) because it is based on occupation and racism. It constantly attacks its neighbors," Mr Ahmadinejad said in a video news conference from New York with the National Press Club in Washington, citing recent Israeli military action in Syria and Lebanon.
"It kills people. It drives people from their homes."
He also took a swipe at the United States, saying: "We oppose the way the US government tries to manage the world We think this method is wrong. It leads to war, discrimination and bloodshed."
But the Iranian leader, who last year railed at the United States in his speech to the General Assembly, played down talk of conflict with the West over Iran's nuclear program, which the United States says it is using to try to build nuclear weapons.
"We think talk of war is a propaganda tool. People who talk have to bring a legal reason for war," he said.