Showing no contrition for defying the world body in March or the declining security situation in Iraq, the US president called for the world to set aside past differences and help rebuild the country: "Now the nation of Iraq needs and deserves our aid - and all nations of goodwill should step forward and provide that support," he said.
But the French president, Jacques Chirac, who spoke after Mr Bush, blamed the US-led war for sparking one of the most severe crises in the history of the UN and argued that Mr Bush's unilateral actions could lead to anarchy.
"No one can act alone in the name of all and no one can accept the anarchy of a society without rules," he said. "The war, launched without the authorisation of the security council, shook the multilateral system. The UN has just been through one of the most grave crises in its history."
Earlier the UN secretary general, Kofi Annan, condemned the doctrine of preemptive military intervention, arguing that it could lead to the unjustified "lawless use of force" and posed a "fundamental challenge" to world peace and stability.
"My concern is that, if it were to be adopted, it could set precedents that resulted in a proliferation of the unilateral and lawless use of force, with or without credible justification," said Mr Annan. "This logic represents a fundamental challenge to the principles on which, however imperfectly, world peace and stability have rested for the last 58 years."
The Brazilian president Luiz Inacio Lula da Silva, who also spoke before Mr Bush, said: "A war can perhaps be won single-handedly. But peace - lasting peace - cannot be secured without the support of all."
Mr Bush's speech was received with polite applause from the 191-member states, while his critics were given a far warmer reception.
The American president was not just under fire for his decision to wage war without international consent but also for his refusal to move more quickly towards handing control of the country back to the Iraqi people.