Barack Obama and his wife Michelle were Tuesday given a full ceremonial welcome at Buckingham Palace on the opening day of their state visit to Britain, DPA reported.
Amid heightened security, the short military ceremony took place in the grounds of Buckingham Palace, rather than on the Mall, the tree-lined avenue leading up to the palace.
After their official welcome by Queen Elizabeth II, and Prince Philip, the US presidential couple met briefly with newly-weds Prince William and his wife, Catherine, before having lunch at the palace.
During his three-day state visit, Obama will visit Westminster Abbey, address both houses of parliament and have political talks with Prime Minister David Cameron.
Obama and his wife brought forward their arrival in London by some 12 hours, flying in late Monday from Ireland to avoid possible air traffic disruption over Britain from the Icelandic volcanic cloud.
The couple will be guests of the queen at Buckingham Palace. But following their unexpected early arrival, they spent their first night at Winfield House, the US ambassador's residence in London.
In a joint article published in the Times newspaper Tuesday, Obama and Cameron reaffirmed the close cooperation between the two countries on international issues, ranging from Afghanistan to the Arab spring.
The two leaders said the traditionally close ties between the US and Britain were "an essential relationship for us and for the world."
Co-operation between Britain and America ranged from Libya to Afghanistan, Pakistan, counter-terrorism and the Middle East peace process, they stressed.
"When the United States and Britain stand together, our people and people around the world can become more secure and more prosperous," the article said.
"Ours is not just a special relationship, it is an essential relationship - for us and for the world."
While founded on a "deep emotional connection," the "natural partnership" was the "perfect alignment of what we both need and what we both believe."
The two leaders also made a point of pledging their support for the protest movements in the Arab world.
"We will not stand by as their aspirations get crushed in a hail of bombs, bullets and mortar fire," they said.
"We are reluctant to use force, but when our interests and values come together, we know we have a responsibility to act...We will stand with those who want to bring light into dark, support those who seek freedom in place of repression, aid those laying the building blocks of democracy."