Iranian President Mahmoud Ahmadinejad was about to speak Tuesday before the U.N. General Assembly, only a few hours after President Bush took the same podium, and a day after his controversial visit to Columbia University in which he criticized the United states and sidestepped harsh words against his own country.

Tuesday morning, Bush issued a "challenge to conscience" to the United Nations, urging its members to defend basic human rights and step up the fight against repressive regimes, including Iran.

Bush backed up his challenge by announcing tough new sanctions against the military rulers of Myanmar, referring to the nation by its pre-junta name, Burma.

"Americans are outraged by the situation in Burma," the president said, accusing the regime of imposing "a 19-year reign of fear" that denies basic freedoms of speech, assembly and worship.

The president delivered a stern lecture on the responsibility of nations to defend all people from oppression.

"Every civilized nation also has a responsibility to stand up for the people suffering under dictatorship," the president said. "In Belarus, North Korea, Syria and Iran, brutal regimes deny their people the fundamental rights enshrined in the Universal Declaration" of Human Rights, a United Nations advisory declaration drawn up almost 60 years ago.

He also singled out Cuba and Zimbabwe as human rights violators, but made scant mention of the war in Iraq or Iran, whose president, Ahmadinejad, was scheduled to speak at the U.N. later in the day.

"The nations in this chamber have our differences, yet there are some areas where we can all agree," Bush said. "When innocent people are trapped in a life of murder and fear, the declaration is not being upheld. When millions of children starve to death or perish from a mosquito bite, we're not doing our duty in the world. When whole societies are cut off from the prosperity of the global economy, we're all worse off."

He said the Declaration of Human Rights demanded action from the members of the world body.

"Changing these underlying conditions is what the declaration calls the work of larger freedom and it must be the work of every nation in this assembly," he said. "This great institution must work for great purposes: to free people from tyranny and violence, hunger and diseases, illiteracy and ignorance, and poverty and despair."

The president also urged the United Nations to send troops to secure the peace in the Sudan's war-ravaged region of Darfur.

"The United Nations must answer this challenge to conscience and live up to its promise to promptly deploy peace-keeping forces to Darfur," Bush said.

He added that in Myanmar, "forced child labor, human trafficking and rape are common." He said the regime is holding more than 1,000 political prisoners, including Aung San Suu Kyi, a Nobel Peace Prize winner.

Bush said the U.S. will tighten economic sanctions on that nation's leaders and financial backers, impose an expanded visa ban on human rights violators from that country, support humanitarian groups working in Myanmar and urge the United Nations "to use their diplomatic and economic leverage to help the Burmese people reclaim their freedom."

The president also noted that the world body must abide by its charter's pledge to stamp out hunger, disease, illiteracy and other challenges to human liberty. He praised the nations of Ukraine, Georgia, Kyrgyzstan, Mauritania, Liberia, Sierra Leone and Morocco as making progress toward liberty.

He said mainstream leaders in the Palestinian territories are working to build free institutions that fight terror, enforce the law and respond to the needs of their people, and should be supported to "advance the vision of two democratic states, Israel and Palestine, living side by side in peace and security."

In his speech, the president proposed a new initiative to alleviate hunger in Africa and elsewhere. It would involve purchasing the crops of local farmers, rather than shipping in food from the developed world.

"This would help build up local agriculture and break the cycle of famine in the developing world. And I urge our United States Congress to support this initiative," Bush said.

While Ahmadinejad was due to speak to the General Assembly late Tuesday, one of his close allies — who caused a stir last year — did not come to New York and said he would not be attending the opening session.

Venezuelan President Hugo Chavez told local radio in Caracas that he doesn't have time in his schedule, and was staying at home in part to meet the parents of hostages being held in Colombia.

Also absent is longtime Cuban tyrant Fidel Castro, who has been ill for more than a year. When Bush mentioned Cuba in his address, saying "the long rule of a cruel dictator is nearing its end," that country's U.N. delegation stood up and walked out of the auditorium.