The 67 participants from 30 countries included former U.S. Ku Klux Klan leader David Duke.
The New York Times reported that Duke, a white supremacist, was expected to claim that Germany built no gas chambers or extermination camps during World War II.
"Depicting Jews as the overwhelming victims of the Holocaust gave the moral high ground to the Allies as victors of the war and allowed Jews to establish a state on the occupied land of Palestine," Duke said, according to the summary of the paper he will deliver, the Times reported.
Duke told BBC cameras in Tehran that he is not a Holocaust denier.
"I'm a Holocaust questioner," he told the BBC. "But I'm here to defend freedom of speech."
Other attendees include a number of Western Holocaust skeptics who have been prosecuted in Europe for publishing their theories casting doubt on whether 6 million Jews were killed by the Nazis or whether gas chambers were ever used.
"The number of victims at the Auschwitz concentration camp could be about 2,007," Australian Frederick Toben told the conference, according to a Farsi translation of his comments. "The railroad to the camp did not have enough capacity to transfer large numbers of Jews."
The two-day conference was initiated by President Mahmoud Ahmadinejad in an apparent attempt to burnish his status at home and abroad as a tough opponent of Israel.
The hard-liner president has described the Holocaust as a "myth" and called for Israel to be wiped off the map. Earlier this year, his government backed an exhibition of anti-Israel cartoons in a show of defiance after Danish cartoons caricaturing Islam's prophet Muhammad were published in Europe, raising an outcry among Muslims.
Organizers and participants touted the conference as a scholarly gathering aimed at discussing the Holocaust away from Western taboos and the restrictions imposed on scholars in Europe. In Germany, Austria and France, it is illegal to deny aspects of the Holocaust.
Duke, a former Louisiana state representative, praised Ahmadinejad for his "courage" in holding a conference "to offer free speech for the worlds most repressed idea: Holocaust revisionism."
"In Europe you can freely question, ridicule, and deny Jesus Christ. The same is true for the prophet Muhammad, and nothing will happen to you," he said in his speech. "But offer a single question of the smallest part of the Holocaust and you face prison."
Also among the participants were two rabbis and four other members of the group Jews United Against Zionism, who were dressed in the traditional long black coats and black hats of ultra-Orthodox Jews. The group rejects the creation of Israel on the grounds that it violates Jewish law.
Rabbi Ahron Kohen urged participants not to deny the Holocaust. "If we say that this crime did not happen, it is a humiliation and insult to the victims," he said, according to the Farsi translation.
But he added that Zionists have used the Holocaust to "give legitimacy to their illegitimate project," the creation of Israel.