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Meeting for the first time since the United States went to war against Iraq (search) without U.N. authorization, world leaders on Tuesday criticized President Bush's policy of "pre-emptive" military strikes and demanded that conflicts and global threats be resolved collectively by all nations.

The fallout from the divisive Security Council battle over the war, which sidelined the United Nations after more than a decade of trying to disarm Saddam Hussein (search), was a focal point of every speech on the opening day of the U.N. General Assembly's annual ministerial debate.

Despite lingering differences over the war, nations rallied behind Secretary-General Kofi Annan (search)'s call to join forces to build a peaceful democracy in Iraq, but sharp differences remained over the timetable and the U.N. role.

French President Jacques Chirac (search), who led the opposition to the war, criticized the United States for launching a war without U.N. approval and undermining the international system of collective security. That system, he said, must now be modernized and restored.

The problems facing the world today -- including Iraq -- can be addressed only in a multilateral forum like the United Nations, which guarantees "legitimacy and democracy, especially in matters regarding the use of force or laying down universal norms," he said.

South African President Thabo Mbeki said Iraq raised questions about "the very future of the United Nations." He said it was a test of whether the world body enjoyed the confidence of the world and was capable of being "the principal guarantor of international peace and security."

"The poor of the world expect an end to violence and war everywhere," Mbeki said. "For us, collectively, to meet these expectations, will require that each and every one of us, both rich and poor ... commit ourselves practically to act. This includes the most powerful."

Annan set the stage for the two-week session, challenging the 191 U.N. member states before they arrived to re-examine the way the international community is dealing with wars, terrorism, poverty and other threats to international security. He urged world leaders not to shy away from recommending "radical" changes -- including to the United Nations.

His call brought 86 presidents and prime ministers, three vice presidents, 99 foreign ministers, and three heads of delegation to U.N. headquarters, a very large turnout for the annual session. Chirac said it "definitely shows that the U.N. is not discredited as some have tried to say or have us believe."

In his keynote address at Tuesday's opening, the secretary-general criticized Bush's "pre-emptive" attack on Iraq, warning that unilateral action could result in a proliferation of unlawful attacks "with or without credible justification."

He underlined that all nations should collectively address the threats that prompt pre-emptive action -- terrorism and weapons of mass destruction.

Indonesian President Megawati Sukarnoputri pointedly began her address with praise and gratitude for the United Nations from the world's largest Muslim nation and strong criticism for "the big powers" for their attitudes toward Islamic countries and for failing to resolve the Israeli-Palestinian conflict.

"The war [in Iraq] has created far many more problems than those it intended to solve," she said. "I do believe that a great many lessons can be learned from the Iraq war."

Rejecting unilateralism, she said all nations "must have the courage to review, revitalize and empower" the United Nations and other regional institutions to strengthen international cooperation.

Brazil's President Luis Inacio Lula da Silva said the impasse over reconstruction in Iraq "can only be overcome under the leadership of the United Nations -- leadership not only in re-establishing acceptable security conditions, but equally in guiding the political process toward the restoration of Iraqi sovereignty as soon as possible."

"We must not shy away from our collective responsibilities," he said. "A war can perhaps be won single-handedly. But peace -- lasting peace -- cannot be secured without the support of all."