Queen greets President Obama on first UK state visit
The Queen has greeted US President Barack Obama, and his wife Michelle, at the start of his first UK state visit.
The Obamas also met other senior royals at Buckingham Palace and later David and Samantha Cameron at Downing Street on day one of the three-day trip.
The couple laid a wreath on the Tomb of the Unknown Warrior in Westminster Abbey and a state banquet at Buckingham Palace is getting under way.
Ahead of the visit David Cameron and Mr Obama spoke of "essential" UK-US 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."
The presidential pair's visit to Westminster Abbey included an impromptu meeting with choirboys, when Mr Obama was teased by his wife for his lack of singing talent.
"He insisted on speaking to each one of them and shaking their hands," the Dean, Dr John Hall said.
"He said that he liked to think he could sing and Mrs Obama said 'Well, he can't really, he can dance'."
Mr Obama briefly met the prime minister in Downing Street ahead of talks on Wednesday. They then left together for a surprise visit to the Globe Academy in Southwark, south London, where they teamed up for a table tennis match against schoolboys.
The leaders' talks are likely to focus on the Middle East and the ongoing conflict in Libya.
In their article, 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.
"We are reluctant to use force, but when our interests and values come together, we know we have a responsibility to act."
BBC diplomatic correspondent Bridget Kendall said the two countries were anxious to play up their closeness.
"Security is the secret glue at the core of the special relationship, and that bond is being strengthened," our correspondent said.
"Behind the flags and formalities, be sure there are tensions, but in this era of tightened budgets and sudden crises, there's a new eagerness to work together."
Mr Obama arrived in the UK from the Republic of Ireland a day ahead of schedule on Monday, to avoid any disruption from a volcanic ash cloud.
After the president and his wife joined the Queen at Buckingham Palace they briefly met the Duke and Duchess of Cambridge before moving to the palace gardens where there was a ceremonial welcome, including a 41-gun salute.
There was an exchange of gifts, with the Obamas presenting the Queen with a collection of memorabilia and photographs from her parents' 1939 visit to the US.
They received a selection of letters from the royal archives, between past US presidents and English monarchs.
When President Obama was shown letters and artefacts charting Britain's loss of the American colonies, he joked: "That was only a temporary blip in the relationship."
Michelle Obama was also given an antique brooch made of gold and red coral in the form of roses.
Number 10 barbecue
Later, the president met Labour leader Ed Miliband at Buckingham Palace.
The two men had a "warm and friendly" meeting, lasting 40 minutes according to Labour sources.
Charles Anson, former press secretary to the Queen, told the BBC the banquet would be a "glittering" affair and one that was "full of warmth" as well as formality.
Guests include Richard Branson, actor Kevin Spacey, the Archbishop of Canterbury and former prime minister Sir John Major.
The BBC's royal correspondent Nicholas Witchell said the Queen and President Obama had a "genuinely good relationship".
"I think the Obamas regard her as a figure with great experience, who's respected around the world, she's known 12 US presidents.
"On her side I think the Queen is now more at ease, more relaxed perhaps than she has been at earlier points in her reign," our correspondent said.
Wednesday's itinerary will include talks with the prime minister before Mr Obama is joined by his wife and British and American military veterans for a barbecue at Number 10.
They will visit the Houses of Parliament and give a speech about US foreign policy to MPs in Westminster Hall, before a return banquet at Winfield House, where the Queen will formally say farewell.