Obama's words of comfort for Brown

After two hours of talks at Downing Street with the Prime Minister, the favourite for the White House said it was much easier to be popular before you were elected to power.

The Democratic senator also emphasised the importance of the so-called "special relationship" with Britain to dealing with global issues such as climate change and economic turbulence.

Dozens of people gathered to chant his name outside the Downing Street gates and around Parliament where he spent a further hour with Conservative leader David Cameron.

But there was none of the "Obamania" seen in Germany when tens of thousands thronged the streets of Berlin to hear him make a keynote speech.

Mr Obama, speaking to a huge crowd of journalists outside Number 10, said he had enjoyed "a terrific conversation" with Mr Brown on the final day of his tour.

"The Prime Minister's emphasis, like mine, is on how we can set up the transatlantic relationship to solve problems that cannot be solved by any country individually: climate change; international terrorism; some of the issues surrounding financial markets," he said.

He said he had not given Mr Brown, still reeling from the devastating Glasgow East by-election defeat, any advice.

But he added: "You're always more popular before you're actually in charge. Once you're responsible then you're going to make some people unhappy. That's just the nature of politics and these things go in cycles."

The US had a "deep and abiding affection" for Britain, he said, and was grateful for its "stalwart assistance" in Iraq and Afghanistan given the "heavy price" in troop losses.