"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.
In the most dominant development, Mr. Obama and Russian President Dmitry Medvedevfollowing a meeting that the two counties reached a deal to reopen nuclear disarmament talks to "pursue" a reduction in the number of warheads held by both countries, which the president called the "gravest threat to humanity."
White House officials say it's part of an ambitious agenda worked out in advance, to give U.S.-Russian relations a fresh start, reports CBS News chief White House correspondent Chip Reid.
"Over the last several years the relationship between our two countries has been allowed to drift," Mr. Obama said.
The flurry of diplomacy came as Mr. 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 Medvedev, British Prime Minister Gordon Brown and Chinese President Hu Jintao - and promptly accepted invitations to visit Russia in July and China later in the year.
The president also took time out to meet visit Buckingham Palace for a private audience with Queen Elizabeth II, to whom he presented an iPod. (Read about the meeting here.)
Thousands of protesters, meanwhile, converged on central London to.
Even as he courted foreign leaders, the president spoke directly to anxious families back home, Mr. 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. Questioned about France and Germany, who have resisted Mr. Obama's calls for more stimulus spending to spur the global economy, the president said those differences have been vastly overstated, reports Reid.
"I am absolutely confident that this meeting will reflect enormous consensus about the need to work in concert to deal with these problems," Mr. Obama said.
The president 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," Mr. 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. (A closer look at the press conference is here.)
|Photos: Obama In London|
The president has his first full day in London. (Photo: AP)
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," Mr. Obama said. (Inside the Obama-Medvedev meeting.)
Earlier Wednesday, the president acknowledged strained relations between the two nations but says the time has come to emphasize areas where they can work together.
The White House said Mr. 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. (Read more on the meeting and the issue here.)
Mr. 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 fighting the recession.
The administration also announced that it will continue high-level talks with China but reduce the frequency to one meeting a year, instead of two. The first meeting under Mr. Obama will be held this summer in the U.S.
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, Mr. Obama also met with Brown's main rival - David Cameron, leader of Britain's opposition Conservative Party.