U.S. President Barack Obama says he seeks unity at the G-20 economic summit in London, and a new arms deal with Russia. It was a busy day of diplomacy for Mr. Obama as he launched his first overseas trip as president.

From a press conference with British Prime Minister Gordon Brown to a round of meetings with world leaders to an audience with Queen Elizabeth, Barack Obama made his presence known on the world stage. "I came here to put forward our ideas. But I also came here to listen and not to lecture," he said.

He began his day at 10 Downing Street, the British prime minister's official residence, amid last minute preparations for the G-20 economic summit.

There have been signs of tension leading up to the meeting, amid differences over the best way to heal the ailing world economy.

But President Obama said he believes there will be a meeting of the minds. "I am absolutely confident that this meeting will reflect enormous consensus about the need to work in concert to deal with these problems," he said.

Prime Minister Brown said it is time for the world's leaders to make difficult decisions. "The truth is that today's global problems require global solutions. And at this week's summit, where leaders representing 85 percent of the world's economy are gathering together, this summit cannot simply agree to the lowest common denominator. We must stand united in our determination to do whatever is necessary," he said.

But talks on the sidelines of the summit are about more than economics.

As he sat down for his first face-to-face meeting with Russian President Dmitri Medvedev, President Obama turned the focus to arms control.

The two men announced the beginning of a new round of negotiations on reducing long-range nuclear weapons.

White House aides called it a major breakthrough. Mr. Obama spoke of a new direction in U.S.-Russia relations. "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 non-proliferation treaties," he said.

President Obama also accepted an invitation to go to Russia in July.

A few hours later, during talks with Chinese President Hu Jintao, he agreed to visit China later in the year.

Speaking through a translator, the Chinese leader expressed his hopes for an enhanced dialogue with the United States. "The Chinese side is willing to work together with the U.S. side to secure even greater progress in the development of the China-U.S. relationship. And I'm willing to establish a good working relationship and personal friendship with President Obama," he said.

At the end of the day, the delegates to the G-20 went to Buckingham Palace for a reception with Queen Elizabeth, a working affair that was short on ceremony.

Meanwhile, in the narrow streets of London's main financial district, there were massive demonstrations, some of them violent. Organizers called the April 1 display of dissent a "Financial Fools Day" protest.