Ref: The Queen has greeted US President Barack Obama, and his wife Michelle, at the start of his first UK state visit.
The Obamas, who are staying at Buckingham Palace, also spent 20 minutes with newlyweds the Duke and Duchess of Cambridge in the 1844 room.
It comes after he and David Cameron spoke of the “essential” US-UK ties.
In a joint article in the Times, Mr Obama and Mr Cameron said of their countries` 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.”
On Wednesday Mr Obama will meet Prime Minister David Cameron for talks.
The leaders are likely to focus on the Middle East and the ongoing conflict in Libya.
They also vowed not to abandon the protesters fighting for democracy in Arab countries, writing that they would “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.
“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.”
Mr Obama arrived in the UK a day ahead of schedule on Monday, to avoid any disruption from a volcanic ash cloud.
The president flew into Stansted, in Essex, with wife Michelle after leaving the Republic of Ireland, where his week-long tour of Europe began.
He and his wife, Michelle, spent the night at US ambassador`s residence Winfield House, in London`s Regent`s Park.
The president`s itinerary includes joining the Duke of Edinburgh to inspect a Guard of Honour in the palace gardens, before a private lunch with the Queen.
Security is tight during the three-day trip which comes ahead of the G8 summit of world leaders in France.
The state visit to the UK is the 101st to be hosted by the Queen but only the third involving a US president in 100 years. The last US leader to come to the UK on one was George Bush in 2003.
The public will get their first chance to see President Obama and the First Lady when they arrive at Westminster Abbey to lay a wreath at the Tomb of the Unknown Warrior.
The couple will then go to Downing Street to meet the prime minister and his wife, Samantha, before returning to Buckingham Palace to meet Labour leader Ed Miliband.
In the evening there will be a state banquet for the Obamas at Buckingham Palace, where they will stay overnight.
On Wednesday, Mr Obama will hold talks with Mr Cameron at Downing Street.
He will be joined later by Michelle for a barbecue at Number 10 with British and American military veterans.
There will be a news conference, before the president departs for the Houses of Parliament, where he will give a speech about US foreign policy to MPs in Westminster Hall.
The day will be rounded off with a return banquet at Winfield House, where the Queen will formally say farewell