White House hopeful Barack Obama has said a strong transatlantic relationship is needed to deal with a wide range of world issues.
Mr Obama said co-operation with the UK was crucial over climate change, terrorism and the economy, after talks with UK Prime Minister Gordon Brown.
He said his conversation with Mr Brown in Downing Street had been "terrific".
The Democratic presidential candidate later met Conservative leader David Cameron at the Houses of Parliament.
Mr Obama and Mr Brown talked for two hours and then took a stroll in the sunshine around Horse Guards Parade before the Illinois senator spoke to reporters.
Heavy price
Speaking outside Downing Street, Mr Obama, who is on the final part of the European leg of his tour, also thanked the British people for their support in the conflicts in Iraq and Afghanistan.
"I know that the troops here in Great Britain have borne a heavy price for wars in both Iraq and Afghanistan and I think the American people are grateful for all the help that has been provided," he said.
"The prime minister's emphasis - like mine - is on how we can strengthen the trans-Atlantic relationship to solve problems that can't be solved by any single country individually," he added.
Mr Obama spoke of a "deep and abiding affection for the British people in America and a fascination with all things British".
He also referred to a shared history and the role of the "English tradition" in shaping the US constitution.
"We've been through two world wars together," he said.
"We speak a common language. We share a belief in rule of law and due process."
Photo opportunity
Earlier, Mr Obama had a breakfast meeting in London with former Prime Minister Tony Blair, who is now the international Middle East envoy.
Mr Obama and Mr Cameron spent almost an hour talking in the Tory leader's Commons office. Shadow foreign secretary William Hague and shadow chancellor George Osborne also attended part of the meeting.
The Tory leader highlighted some of Parliament's features as they posed outside for photographs before the talks.
A Conservative party spokeswoman said their conversation had focused on Afghanistan - including a comparison of their recent personal visits to the country - Iraq, Iran and the economy.
Mr Obama has already visited Germany, where he gave a keynote foreign policy speech in Berlin, and France where he met President Nicolas Sarkozy.
After flying into London on Friday, Mr Obama was greeted by the American ambassador and his wife, Robert and Maria Tuttle, before being taken to a city centre hotel.
In contrast to the public reception he received in Berlin and Paris, Mr Obama's London visit was kept deliberately low key.
Opinion polls
In Paris, Mr Obama said Iran should not wait for the next US president to be elected before resolving its dispute with the West.
"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"
He said Tehran should promptly accept an international call to freeze its "illicit nuclear programme".
Iran insists its nuclear campaign is peaceful.
Mr Obama's tour has also taken in the Middle East where he visited Jordan, Israel and the Palestinian territories.
The senator left London in the afternoon on a chartered plane to fly back to the US where he faces Republican rival John McCain in November's presidential election.
The latest opinion polls in the US show Mr Obama leads Mr McCain by between one and six percentage points with some polls showing the race tightening in key states.
Mr Obama said he was unsure what political impact his overseas trip would have among US voters concerned about fuel prices and their mortgages.
He told reporters in London: "I wouldn't even be surprised that in some polls you saw a little bit of a dip as a consequence. We've been out of the country for a week."
He added: "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."
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