Ayatollah Ali Khamenei, the country's supreme leader, also used a pro-Palestinian conference in Tehran to rally support from Islamic nations for the cash-strapped, Hamas-led Palestinian government.
Mahmoud Ahmadinejad, the Iranian president, dismissed remarks by Condoleezza Rice, the American secretary of state, who said the United Nations must consider strong action to press Iran to comply with demands over its nuclear plans.
"What she said is not important. She is free to speak out," Ahmadinejad said on the sidelines of a conference in Tehran, the official Islamic Republic News Agency reported on Friday.
Ahmadinejad drew condemnation from the US and other world powers by announcing on Tuesday that Iran had enriched uranium to a level used in power stations, defying UN demands for it to halt enrichment.
The president also told a gathering of Iranian officials, visiting Palestinian leaders and foreign supporters that "the Zionist regime is an injustice and by its very nature a permanent threat".
"Whether you like it or not, the Zionist regime is on the road to being eliminated," added Ahmadinejad, who drew international condemnation last year when he said Israel should be "wiped off the map".
"If there is serious doubt over the Holocaust, there is no doubt over the catastrophe and holocaust being faced by the Palestinians"
But unfazed by his critics in Europe and Washington - who are also piling the pressure on Iran over its disputed nuclear drive - he went on to repeat his controversial stance on the Holocaust.
"If there is serious doubt over the Holocaust, there is no doubt over the catastrophe and holocaust being faced by the Palestinians," said the president, who had previously dismissed as a "myth" the killing of an estimated six million Jews by the Nazis and their allies during the second world war.
"I tell the governments who support Zionism to ... let the migrants (Jews) return to their countries of origin. If you think you owe them something, give them some of your land," he said.
The Iranian government does not recognise Israel.
In his speech on Friday, Ayatollah Khamenei accused the United States of conspiring to place the entire Middle East under Israeli control.
"The plots by the American government against Iran, Iraq, Syria and Lebanon aimed at governing the Middle East with the control of the Zionist regime will not succeed," he said.
Khamenei condemned the US
over Iraq and Guantanamo
"If, by accident, the American government saw reason, it would respect the wish of the Iraqi people to form its government, respect the Palestinian government, free the prisoners of Guantanamo and Abu Ghraib, cease the conspiracy and not create tension in the Persian Gulf region," he said.
He also called on Muslim countries to help the Hamas-led Palestinian government following a decision by the US and EU to suspend aid.
"The Islamic world cannot remain indifferent and silent to tyranny," he asserted.
"Your martyrs are our martyrs. Your pain is our pain," he said of the Palestinians, adding that "Islamic nations have the duty to help you in every possible way".
EU foreign ministers on Monday formally backed plans for a temporary aid suspension to the Palestinian government, having called on Hamas to renounce violence, recognise Israel and abide by previous Palestinian Authority commitments.
Meshaal says he is waiting for
Iran's answer to his aid request
Speaking to reporters in Tehran, Khalid Meshaal, the Syria-based Hamas leader, said: "It is immoral and inhumane to collectively punish a nation which practises democracy.
"But the Palestinian nation will not give in."
He added that he was talking with Iran as well as Arab countries in a bid to fill Palestinian Authority coffers.
"We are waiting for the Iranian government's support and answer to our request, but we haven't received anything yet."