LONDON -- On the eve of a global economic summit, President Barack Obama promised world leaders he would listen, not lecture, as they seek a common fix to the financial crisis. "We can only meet this challenge together," he said Wednesday as the U.S. and Russia spoke on the summit sidelines about nuclear warhead reduction. 

The flurry of diplomacy came as Obama stepped on the world stage for the first time as president, aiming to shore up both America's economy and its reputation across the globe. He met with British Prime Minister Gordon Brown, Russian President Dmitry Medvedev and Chinese President Hu Jintao -- and promptly accepted invitations to visit Russia in July and China sometime in the second half of the year. 

Thousands of protesters converged on central London to rally against the economic summit. 

In the most dominant development, Obama and Medvedev announced talks to limit the number of nuclear warheads, the first major negotiations in years over what Obama called the "gravest threat to humanity." 

Meanwhile, speaking directly to anxious families back home, Obama sought to restore consumer confidence and encourage people to think about spending now to help their future. 

"Basing decisions around fear is not the right way to go," he said. "We are going to get through this difficult time." 

The president also disputed criticism that the United States was feuding with other nations about the need to pump more money into economic stimulus policies. 

"I am absolutely confident that this meeting will reflect enormous consensus about the need to work in concert to deal with these problems," Obama said. 

Obama prodded nations to spur growth and work together on regulatory reform, and not fall into the kind of protectionism and other mistakes that helped fuel the Great Depression. 

"That is a mistake that we cannot afford to repeat," Obama said during a news conference with Brown, the British leader. 

Brown, too, sought to play up consensus. "We are within a few hours, I think, of agreeing a global plan for economic recovery and reform," he said. 

On Russia, the White House said the U.S. and its former Cold War foe are determined to reduce the number of nuclear warheads but have not settled on a specific number. 

"I think people on both sides of the Atlantic understand that as much as the constant cloud -- the threat of nuclear warfare -- has receded since the Cold War, that the presence of these deadly weapons, their proliferation, the possibility of them finding their way into the hands of terrorists continues to be the gravest threat to humanity," Obama said. 

The discussion centers on a replacement of the dying 1991 Strategic Arms Reduction Treaty, which limited the world's two largest nuclear arsenals. START expires Dec. 5. Both nations say they want to have a new agreement in place before then. 

Earlier Wednesday, Obama acknowledged strained relations between the two nations but says the time has come to emphasize areas where they can work together. 

Obama also gave focus to China. 

The White House said Obama and Hu agreed to "intensify coordination and cooperation on global economic and financial issues." As economic leaders, the United States and China say they want to work together to address the economic crisis. 

Officials in both Russia and China have called for a new global currency to end the dollar's dominance, although it's unlikely that the idea will gain immediate traction. 

Obama's talks with Hu were sure to address Beijing's concerns about the safety of its position as Washington's biggest foreign creditor, with about $1 trillion in U.S. government debt. For the U.S., there are fears that any Chinese flight away from those investments would erode the U.S. ability to spend more on recession-fighting. 

The G-20 meetings open with a working dinner Wednesday night and continue throughout Thursday. 

Before the summit even opened, divisions emerged among nations' leaders. 

French President Nicolas Sarkozy has expressed dissatisfaction with how the summit has been shaping up and warned he would rather risk a public dispute than accept a vague consensus or a "false compromise." 

Brown said he was confident Sarkozy would still be at the table when the dinner was complete.
In London, Obama is also meeting with Brown's main rival -- David Cameron, the leader of Britain's opposition Conservative Party. During the early evening hours, he and first lady Michelle Obama headed to Buckingham Palace for an audience with Queen Elizabeth II. 

Thousands of well-wishers gathered in front of the palace to wave as the presidential limo passed.