U.S. President Barack Obama delivers his first speech to the UN General Assembly in New York on Wednesday. (CBC)World leaders must work together to address global challenges in the 21st century because "power is no longer a zero sum game," U.S. President Barack Obama told the UN Wednesday.

"Nothing is easier than blaming others for our troubles and absolving ourselves of responsibility for our choices and our actions. Anyone can do that," he said in his first speech to the United Nations General Assembly in New York. "Responsibility and leadership in the 21st century demand more."

While Obama's speech stressed co-operation, Wednesday's speeches from world leaders at the assembly provided a window into the divisiveness that remains a hallmark of the international organization.

Numerous countries, including Canada, are expected to walk out of the assembly ahead of Iranian President Mahmoud Ahmadinejad's speech to protest the lack of human rights in Iran, while other countries have said they will walk out if he denies the Holocaust.

Representatives from many countries, including the United States, also walked out before Libyan leader Moammar Gadhafi delivered a rambling, 96-minute speech blasting the UN for failing to intervene or prevent some 65 wars around the world.

'Reflexive anti-Americanism'

For Obama, his speech was a chance to wash away any lasting images of U.S. unilateralism under predecessor George W. Bush, and he said he used it as an opportunity to renew engagement with the world, which he said has entered a new era where "our destiny is shared."

"I took office at a time when many around the world had come to view America with skepticism and distrust" due to opposition to specific policies and U.S. unilateral action on others, Obama said.

The world has been in a state of "reflexive anti-Americanism, which too often has served as an excuse for our collective inaction," he said.

Libyan leader Moammar Gadhafi addressed the 64th session of the United Nations General Assembly on Wednesday and called for the abolishment of the veto power of the five permanent members. (Richard Drew/Associated Press)

Obama highlighted the challenges facing the world, including al-Qaeda and other extremist groups, the development and security of Pakistan and Afghanistan, ending the war in Iraq, nuclear proliferation, the Middle East peace process, climate change and the global economy.

Obama's told leaders that they must shoulder more responsibility in resolving global problems.

"This cannot be solely America's endeavour," he said.

Inequality among member states: Gadhafi

Following Obama's speech, which had praised the UN's peacekeeping efforts, Libyan leader Moammar Gadhafi made his first appearance at the general assembly, attacking what he called the inequality among UN member states.

He called for the reform of the Security Council — abolishing the veto power of the five permanent members or expanding the body with additional member states to make it more representative.

"There is no respect for the United Nations, no regard for the General Assembly," said Gadhafi, who at one point tore up a copy of the UN Charter.

Gadhafi's message was somewhat lost in the confusing narrative of his speech, in which he went on to question whether conspiracies were behind the assassination of key political figures in history, including John F. Kennedy and Martin Luther King Jr. in the United States.

He also demanded that vaccines be given free to the children of the world, and suggested that capitalist countries had a conscious role in the creation of viruses such as the swine flu.

Former Cuban leader Fidel Castro's 4½-hour-long appearance in 1960 holds the record for the longest speech in front of the General Assembly.

Sarkozy calls for expansion of Security Council

Gadhafi's speech drew a strong rebuke from British Prime Minister Gordon Brown, who began his speech before the assembly saying, "I stand here to reaffirm the United Nations charter, not to tear it up."

But Gadhafi's call for Security Council reform received a sympathetic ear from French president Nicolas Sarkozy, who called on the UN to broaden the membership of the Security Council and other international institutions to reflect the changes in the world since the end of the Second World War.

The United States, Russia, China, Britain and France are the five permanent members on the 15-member council.

Italy's President Silvio Berlusconi sounded a note of caution to France's proposal, saying letting more permanent members into the council would only act to further isolate those countries that are not members.

Nations to walk out on Ahmadinejad

Russian president Medvedev, Zimbabwean President Robert Mugabe and Ahmadinejad are also scheduled to address the assembly on Wednesday.

Ahmadinejad told The Associated Press on Tuesday he would be urging the U.S. and other countries to embrace Iran as a friend rather than an enemy.

In his speech, Obama said if Iran and North Korea continue to pursue nuclear weapons at the cost of regional stability and security, they must be held accountable by the UN.

But Ahmadinejad said Iran will not be giving up its nuclear program, and he expects "free and open" discussions with leaders about "the sources of insecurity around the world."

Foreign Affairs Minister Lawrence Cannon will speak on Canada's behalf on Friday.

With files from The Associated Press