Barack Obama and his wife Michelle are in Britain for a three-day state visit. The President and First Lady were welcomed to Buckingham Palace by the Queen and the Duke and Duchess of Cambridge.
The American leader has a busy round of political talks and meetings during his high profile three-day stay, as well as a lavish state banquet.
President Obama and his wife Michelle were greeted at Buckingham Palace by the Queen before spending around 20 minutes with the Duke and Duchess of Cambridge.
It was the first time Kate had undertaken a role as a member of the royal family since the wedding. There were no television cameras nor reporters in the room for the meeting.
The state visit is only the third by a US president to the UK in 100 years and comes at a time of close co-operation between Britain and America on issues ranging from Libya to Afghanistan, counter-terrorism and the Middle East peace process.
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The US President flew into Stansted Airport from Ireland ahead of schedule on Monday night to avoid the possibility of disruption caused by the ash cloud from an erupting Icelandic volcano.
After lunch with the Queen at Buckingham Palace Barack Obama laid a wreath at the Grave of the Unknown Warrior at Westminster Abbey
The Obamas are then going Downing Street to visit the Prime Minister David Cameron and his wife Samantha before meeting Labour leader Ed Miliband at Buckingham Palace.
Barack Obama will hold talks with the Prime Minister on Wednesday, with Libya and the Middle East likely to dominate discussions.
Writing a joint article in the Times, Mr Obama and Mr Cameron talked of the UK-US relationship: “Ours is not just a special relationship, it is an essential relationship – for us and for the world.
“When the United States and Britain stand together, our people and people around the world can become more secure and more prosperous.
“The reason it thrives is because it advances our common interests and shared values. It is a perfect alignment of what we both need and what we both believe.”
They also promised not to abandon the protesters fighting for democracy in the “Arab Spring”.
“We will not stand by as their aspirations get crushed in a hail of bombs, bullets and mortar fire,” said the two leaders.
“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.”