World leaders were on Wednesday to step into the global spotlight for the UN's annual diplomatic dance, with US President Barack Obama set to warn that America cannot solve the planet's problems alone.

With several firebrand leaders also due to step up to the podium at the UN General Assembly, the political pow-wow is fraught with the potential for unwanted public encounters between foes.

Israel has already urged other nations to boycott a speech by Iranian leader Mahmoud Ahmadinejad for his repeated denial of the Holocaust.

And Libya's President Moamer Kadhafi, known for his long, rambling speeches on the history of the Arab people, usually delivered from his nomad tent, will step up to the UN podium for the first time in his four-decade rule.

But all eyes will be on Obama addressing the UN General Assembly for the first time since taking office in January, after the George Bush administration was accused of riding roughshod over the 192-member global body.

"Those who used to chastise America for acting alone in the world cannot now stand by and wait for America to solve the world's problems alone," Obama tells the assembly in extracts released by the White House.

"We have sought -- in word and deed -- a new era of engagement with the world. Now is the time for all of us to take our share of responsibility for a global response to global challenges."

Kadhafi, who toured the UN building late Tuesday, was to directly follow Obama onto the podium, in what could be a delicate moment for the US leader amid a row over the release of the Lockerbie bomber, a former Libyan spy.

And Obama was to warn the world would pay a heavy price if it fails to act against militants.

"Consider the course that we are on if we fail to confront the status quo. Extremists sowing terror in pockets of the world. Protracted conflicts that grind on and on," the president says.

"Genocide and mass atrocities. More and more nations with nuclear weapons. Melting ice caps and ravaged populations. Persistent poverty and pandemic disease.

"I say this not to sow fear, but to state a fact: the magnitude of our challenges has yet to be met by the measure of our action."

UN chief Ban Ki-moon will also seek to set the tone by appealing in his opening address for "genuine"collective action by world leaders to roll back climate change as well as global poverty.

"If ever there were a time to act in a spirit of renewed multilateralism -- a moment to create a United Nations of genuine collective action -- it is now," Ban says in remarks prepared for delivery.

Ahmadinejad's speech is keenly awaited as the United Nations grapples with how to resolve a row over its suspect nuclear program and amid international concern over the disputed June elections which returned him to power.

Israel has called for a symbolic protest against Ahmadinejad, but it was not yet clear whether many countries would comply.

"The simple fact of leaving the room during his speech, or not to be present during it, is a symbolic act," Israel's UN ambassador Gabriela Shalev told army radio.

The rising temperatures inside the UN headquarters may be matched by protests on the streets outside, with organizers saying thousands are set to rally "to warn against the dire implications of offering diplomatic and political incentives to the Iranian regime."

President Luiz Inacio Lula da Silva of Brazil, who has pledged to champion the cause of emerging democracies, China's Hu Jintao and Russian President Dmitry Medvedev also take to the floor on Wednesday.