* Obama calls for more help with global problems
* Ahmadinejad, other U.S. critics, also due to speak
* Libya's Gaddafi rambles in maiden U.N. speech (Recasts, adds Lula, Russian comments)
UNITED NATIONS, Sept 23 - U.S President Barack Obama told world leaders on Wednesday to stop blaming America and join him in confronting challenges like Iran and North Korea's nuclear programs and the war in Afghanistan.
Leaders of Libya and Brazil, speaking at the annual U.N. General Assembly gathering, both questioned the world's political and economic balance, reflecting deep unease exacerbated by the global economic crisis.
Obama, in his first speech to the assembly since taking office in January, pledged U.S. global engagement but said the United States could not shoulder the responsibility alone.
"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 said.
The U.S. leader, enjoying a global spotlight, urged international leaders to move beyond "an almost reflexive anti-Americanism, which too often has served as an excuse for collective inaction."
Obama, who will host a Group of 20 nations summit in Pittsburgh this week, also pledged to work with allies to strengthen financial regulation to "put an end to the greed, excess and abuse that led us into disaster."
Obama was among the first major speakers at the gathering, which brings more than 100 heads of state and government together to air issues ranging from nuclear proliferation and international terrorism to climate change and global poverty.
Iranian President Mahmoud Ahmadinejad, Venezuelan President Hugo Chavez and Zimbabwean President Robert Mugabe -- all critics of U.S. foreign policy -- are due to address the meeting, guaranteeing a challenge to Obama's world view.
U.N. Secretary General Ban Ki-moon, opening the meeting, urged delegates to put their differences behind them.
"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," he said.
Obama has brought a new tone in U.S. foreign policy, stressing cooperation and consultation over the unilateralism of his predecessor, George W. Bush.
But while the applause he received at the United Nations was testament to Obama's global popularity, the new approach has delivered few concrete foreign policy achievements.
Obama used his speech to sketch out his foreign policy wish list, ranging from encouraging support for the U.S. stance on the war in Afghanistan and nuclear standoffs with Iran and North Korea to Middle East peacemaking, all issues on which he has made little headway so far.
Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas are both attending this week's U.N. meetings. But they have already denied Obama a diplomatic coup he had hoped for -- rebuffing his efforts to reinvigorate stalled Middle East peace talks in time for the U.S. leader's U.N. debut.
Brazilian President Luiz Inacio Lula da Silva, expected to be a key player at the G-20 Pittsburgh meeting, said it was time to rethink the global economic balance of power.
"A senseless way of thinking and acting, which dominated the world for decades, has proved itself bankrupt," Lula said of economic models that discourage regulation.
Libyan leader Muammar Gaddafi made his own U.N. debut amid raw U.S. emotions over the Lockerbie bombing after Scotland's release of a Libyan official convicted in the 1988 attack.
But Gaddafi's rambling 1-1/2 hour speech, which touched on everything from the U.N. charter to the 1963 assassination of former U.S. President John F. Kennedy, ended up driving some delegates from the room in boredom.
More excitement was expected from Iran's Ahmadinejad, whose speech later on Wednesday will likely be the sharpest counterpoint to Obama's address.
Ahmadinejad recently drew fresh international condemnation for calling the Holocaust a lie and repeating Tehran's vow never to bargain away its nuclear program ahead of talks next month with the United States and other powers concerned about Iran's nuclear ambitions.
A senior Russian official said Moscow is ready to discuss further sanctions against Iran if U.N. nuclear inspectors declare it has not fulfilled its commitments.
(additional reporting by Lou Charbonneau, Arshad Mohammed, Claudia Parsons, Caren Bohan, Jeff Mason, Terry Wade, Oleg Shchedrov, and Haitham Haddadin)