Ahmadinejad also asked the conference: "What were the root causes of the US attacks against Iraq or invasion of Afghanistan?
"The Iraqi people have suffered enormous losses ... wasn't the military action against Iraq planned by the Zionists ... in the US administration, in complicity with the arms manufacturing companies?"
Many delegates who remained in the hall applauded Ahmadinejad's comments.
At least three demonstrators, dressed as clowns and shouting "racist, racist," were expelled as Ahmadinejad began to speak.
Alan Fisher, Al Jazeera's correspondent at the conference, said Ahmadinejad had reiterated his views on Israel, especially over its 22-day war on Gaza.
He said: "At the time [of the offensive] he said what was going on in Gaza was a genocide ... this was an opportunity for him to say that at a world forum.
"There are people in the hall who believe that what Ahmadinejad was saying is correct - that is why there is such a split here."
The US, Canada, Israel, Australia, New Zealand, Germany, Poland and the Netherlands, had earlier said they would not attend the conference amid fears Ahmadinejad would use the summit to propagate anti-Semitic views.
Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said: "Ahmadinejad's words are being criticised in Iran, not just among the youth, but among the different political factions.
"This is the exact attitude [for which] he has been criticised for some time."
|Ahmadinejad's speech divided delegates gathered at the UN conference [AFP]
"Even among the conservatives they have said such remarks are totally uncalled for."
Israel's foreign ministry criticised the decision to invite Ahmadinejad to the conference.
“The event's organisers allowed a Holocaust denier that implements a policy of persecution of minorities in his country to open the conference," it said in a statement.
"The Iranian president's virulent incitement and disgraceful racism... are a clear testimony, for whoever still needs any, that the conference's agenda has been hijacked."
Robert Gibbs, White House spokesman, said that Barack Obama, the US president, was strongly opposed to Ahmadinejad's comments.
"This is hateful rhetoric. It is, I think, one of the reasons why you saw the administration and the president determined that its participation in this conference was not a wise thing to do."
Nicolas Sarkozy, the French president, condemned Ahmadinejad's "speech of hate" and called for a "firm and united" reaction from the European Union.
Jonas Gahr Store, Norway's foreign minister, said the Iranian leader's comments had "run counter to the very spirit of dignity of the conference ... he made Iran the odd man out".
The speech by Ahmadinejad, who is a frequent critic of Israel and has cast doubt on the extent of the killing of Jews during the Second World War, coincided with Holocaust Remembrance Day in Israel, which begins at sundown on Monday.
Washington also said it believed a draft text to be discussed was overly critical of Israel and its treatment of the Palestinians.
Opening the five-day summit earlier, Ban Ki-Moon, the UN secretary general, said he was "profoundly disappointed" that some western countries were not attending, but also condemned those who sought to deny or minimise the extent of the Holocaust.
He said: "Some nations who by rights should be helping us to forge a path to a better future are not here ... I deeply regret that some have chosen to stand aside."
Israel had withdrawn its ambassador to Switzerland in protest over a meeting between Ahmadinejad and Hans-Rudolf Merz, his Swiss counterpart.
The UN organised the summit to help heal the wounds left by its last racism conference in Durban, South Africa, in 2001, when the US and Israel walked out after Arab states sought to define Zionism as being racist.
Barack Obama, the US president, announcing his administration's decision not to attend the conference, said Washington wanted a "clean slate" before tackling race and discrimination issues at the UN.
Several Muslim nations at the summit called for moves to prevent perceived insults to Islam, which they say have proliferated since the attacks on the US on September 11, 2001.