UNITED NATIONS —

Iran's Mahmoud Ahmadinejad accused "a few bullying powers" of trying to thwart the country's nuclear programs and vowed to defend Iran's right to peaceful nuclear power, in an address Tuesday afternoon before the United Nations.

The controversial Iranian president stressed that his country was prepared for a dialogue with world powers but refused to accept any illegal demands on Iran's nuclear program.

With major world leaders watching, and just eight hours after President Bush's final speech before the general assembly, Ahmadinejad used the U.N. platform to decry a "Zionist regime," a thinly veiled reference to Israel, for what he described as persecution of other Middle Eastern peoples.

"The Zionist regime is on a definite slope to collapse, and there is no way for it to get out of the cesspool created by itself and its supporters," Ahmadinejad said of Israel.

The Iranian president is feared and reviled in Israel because of his repeated calls to wipe the Jewish state off the map, and his aggressive pursuit of nuclear technology has only fueled Israel's fears.

And in a direct attack on the U.S., he declared that the "American empire in the world is reaching the end of its road," and urged the next presidential administration to "limit their interference to their own borders."

The Iranian president also criticized the American people for having "no respect for the people of Iraq," and said that the war in Iraq was waged under false pretense.

He said that six years after Saddam Hussein's regime was ousted in Iraq, "the occupiers are still there."

"Millions have been killed or displaced, and the occupiers, without a sense of shame, are still seeking to solidify their position in the ... region and to dominate oil resources," Ahmadinejad said.

In Afghanistan, terrorism is spreading quickly and the presence of NATO forces has contributed to a huge increase in the production of narcotics, Ahmadinejad said.

Earlier Tuesday, with Ahmadinejad watching, President Bush urged the international community to stand firm against the nuclear ambitions of Iran and North Korea.

"A few nations, regimes like Syria and Iran, continue to sponsor terror," Bush said. "Yet their numbers are growing fewer, and they're growing more isolated from the world. As the 21st century unfolds, some may be tempted to assume that the threat has receded. This would be comforting. It would be wrong. The terrorists believe time is on their side, so they've made waiting out civilized nations part of their strategy. We must not allow them to succeed."

At one point during Bush's 22-minute speech, Ahmadinejad turned to Iranian Foreign Minister Manouchehr Mottaki and gave a thumb's down.

The criticism of the West made by Ahmadinejad's speech Tuesday seemed to conflict with earlier statements he made during an interview with National Public Radio, wherein he argued that he does not want confrontation with the United States.

In the hours leading up to his speech, Ahmadinejad blamed U.S. military interventions around the world in part for the collapse of global financial markets ahead of his speech Tuesday to the U.N. General Assembly.

Ahmadinejad also said the campaign against his country's nuclear program was solely due to the Bush administration "and a couple of their European friends."

"The U.S. government has made a series of mistakes in the past few decades," Ahmadinejad said an interview with the Los Angeles Times. "The imposition on the U.S. economy of the years of heavy military engagement and involvement around the world ... the war in Iraq, for example. These are heavy costs imposed on the U.S. economy.

"The world economy can no longer tolerate the budgetary deficit and the financial pressures occurring from markets here in the United States, and by the U.S. government," he added.

The U.S. and its allies allege Iran wants to develop its uranium enrichment program to make nuclear weapons. Iran insists its nuclear program is purely peaceful and designed to produce electricity for civilian use.

Iran already is under three sets of sanctions by the U.N. Security Council for refusing to suspend uranium enrichment.

Despite U.N. sanctions against Iran over its nuclear program, Ahmadinejad claimed vast international support for his position and said the campaign consisted "of only three or four countries, led by the United States and with a couple of their European friends."

Iran insists its nuclear activities are geared only toward generating power. But Israel says the Islamic Republic could have enough nuclear material to make its first bomb within a year. The U.S. estimates Tehran is at least two years away from that stage.

Washington and its Western allies are pushing for quick passage of a fourth set of sanctions to underline the international community's resolve.

Democratic Presidential Nominee Barack Obama condemned Ahmadinejad's comments in a statement Tuesday and criticized his opponent John McCain for failure to support "a bipartisan bill to increase pressure on [Tehran's] regime by allowing states and private companies to divest from companies doing business in Iran."

"The threat from Iran's nuclear program is grave," Obama said. "Now is the time for Americans to unite on behalf of the strong sanctions that are needed to increase pressure on the Iranian regime. The security of our ally Israel is too important to play partisan politics"

The Associated Press contributed to this report.