The presidential contender defended his decision to travel to Europe and the Middle East on Saturday, saying that problems encountered by Americans at home are often best dealt with by working with allies overseas.
Obama, who spoke to reporters after wrapping up talks with British Prime Minister Gordon Brown, said he wasn't sure if there would be any immediate political impact from the trip.
"The reason that I thought this trip was important is that I am convinced that many issues that we face at home are not going to be solved as effectively unless we have strong partners abroad," Obama said.
The Democratic hopeful seemed relaxed as he strolled down to the prime minister's office at 10 Downing St., pausing briefly to shake the hands of two somewhat startled police officers standing near the door.
He turned to television cameras, smiled, waved and said "Hello," before walking into Number 10. Brown greeted him just inside the door.
Pooled television images showed Brown offering Obama a chair on the Downing Street terrace before the pair settled down for two hours of talks.
The pair later took a stroll in the sunshine around Horse Guards Parade, the vast open space where military reviews are often held. Tourists snapped pictures while security guards walked ahead of the two men.
The stroll in the vast arena offered a photo opportunity with a London backdrop for Obama, whose visit to London has been decidedly low-key, particularly after the huge crowds he drew earlier in the week in Germany.
Earlier, he met with former Prime Minister Tony Blair, who is now a Middle East envoy. The meeting lasted for just over an hour.
