LONDON -- President Obama plunged into a whirlwind of international diplomacy Wednesday on the eve of the G-20 summit, meeting with world leaders throughout the day and closing the afternoon with a private meeting at Buckingham Palace.
Mr. Obama pledged to lead the world out of its economic crisis and agreed in his first meeting with Russian President Dmitry Medvedev to seek a new treaty on nuclear disarmament before the end of the year.
He also pledged closer economic cooperation with China and rejected suggestions of an unworkable divide between G-20 leaders.
Mr. Obama began the day at a joint press conference with British Prime Minister Gordon Brown, saying the economic crisis presented an opportunity for leadership that the United States would seize, along with other countries.
"Basing decisions around fear is not the right way to go," he said. "We are going to get through this difficult time."
Outside, thousands of protesters rallied in the streets against capitalism and homelessness and demanded more environmentally friendly initiatives.
Large groups smashed windows, entered the Royal Bank of Scotland building and wrestled with police at barricades.
Protesters have directed much of their anger at the United States, where they see bankers as greedy and the federal government as remiss for not being vigilant enough over U.S. financial markets.
The summit marks Mr. Obama's first time on the international stage, where he is expected to face criticism from inside the meetings, too. Critics say Mr. Obama has used too much U.S. taxpayer money to spend his country out of the global recession and that they will not be forced to do the same.
The most vociferous opposition to calls by the United States and Britain to increase fiscal stimulus packages is coming from France and Germany.
"As things stand at the moment, these projects do not suit France or Germany," French President Nicholas Sarkozy told Europe 1 radio.
Mr. Obama, whose administration has promised to inject new life into strained U.S.-Russia relations, met in the afternoon with Mr. Medvedev.
"As I've said in the past, I think that over the last several years the relationship between our two countries has been allowed to drift," Mr. Obama said. "And what I believe we've begun today is a very constructive dialogue that will allow us to work on issues of mutual interest, like the reduction of nuclear weapons and the strengthening of our nonproliferation treaties; our mutual interest in dealing with terrorism and extremism that threatens both countries; our mutual interest in economic stability and restoring growth around the world; our mutual interest in promoting peace and stability in areas like the Middle East."
The president also said he accepted Mr. Medvedev's invitation to visit Moscow in July.
They will begin efforts to improve relations by replacing an existing arms control treaty -- the 1991 Strategic Arms Reduction Treaty -- before it expires in December, with a shared aim of reducing nuclear warheads to roughly 1,500 each from the existing limit of 2,200.
Mr. Medvedev told reporters: "In the past years, there were strains in relations between our two countries, and they were drifting in the wrong direction."
Mr. Obama and first lady Michelle Obama met privately with Queen Elizabeth II at Buckingham Palace and were expected to attend an official summit dinner with other world leaders in the evening.
The Obamas were greeted outside the palace by thousands of well-wishers. They gave the queen a gift of an iPod loaded with video footage and photographs of her 2007 visit to the Virginia cities of Richmond, Jamestown and Williamsburg.
In return, the queen gave the president a silver framed signed photograph of herself and the Duke of Edinburgh -- apparently a standard present for visiting dignitaries, according to the newspaper the Guardian.
It is believed the queen already has an iPod, a 6GB silver Mini version she is said to have bought in 2005 at the suggestion of Prince Andrew, the paper also reported.
Mr. Obama also held his first face-to-face meeting with Chinese President Hu Jintao.
The leaders made only general comments and issued a joint statement vowing to build "a positive, cooperative, and comprehensive relationship for the 21st century."
The statement also said the two leaders agreed on the need to give international financial institutions such as the International Monetary Fund "more resources to help emerging market and developing nations withstand the shortfall in capital." They also "agreed on the need for sweeping changes in the governance structure of international financial institutions," the statement said.
Mr. Obama said the relationship between the United States and China has become "extremely constructive."
Said Mr. Hu: "As President Obama rightly said just now, sound China-U.S. relationship is not only in the fundamental interest of our two peoples and our two countries, but also contributes to peace, stability and prosperity in the Asian Pacific region and in the world at large."
They also agreed to meet in Washington this summer.
There were also signs that relations between the two militaries are getting back on track following China's decision to suspend ties last year in response to U.S. arms sales to Taiwan.
The U.S. chief of naval operations, Adm. Gary Roughhead, will visit China in April to attend events commemorating the 60th anniversary of the founding of the navy of the Chinese People's Liberation Army, the joint statement said.
Mr. Obama and Mr. Brown sought to downplay the scale of disagreements between G-20 leaders that threaten to derail the summit.
"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 separation between the various parties has been vastly overstated."
Continuous News Editor Joseph Weber reported from Washington.