Queen Elizabeth hailed the US as Britain's most important ally tonight at a state banquet in honour of US president Barack Obama and his wife Michelle.
“I firmly believe that the strength of our links and many shared interests will continue to ensure that when the United States and the United Kingdom stand together, our people and other people of goodwill around the world will be more secure and more prosperous,” the Queen said on the first day of Mr Obama's two-day state visit.
Mr Obama thanked the UK for its solidarity since the September 11 attacks and in tackling the security threats that followed.
He also paid tribute to the UK's military forces for "standing shoulder to shoulder with the US for decades".
Around 170 guests including US Secretary of State Hillary Clinton and Prime Minister David Cameron and his wife Samantha listened to the Queen's address in the grand Buckingham Palace ballroom.
Actors Tom Hanks and Kevin Spacey, actress Helena Bonham-Carter and her director husband Tim Burton added some Hollywood glitz to the VIPs attending the lavish white tie dinner.
Former Prime Ministers Sir John Major, Gordon Brown and Tony Blair with wife Cherie were also invited, but the newlywed Duke and Duchess of Cambridge, who earlier met the Obamas, were not there.
Earlier Mr Obama received a royal 41-gun salute at Buckingham Palace.
Ceremonial cannon sounded the blasts as Queen Elizabeth, husband Prince Philip, heir Prince Charles and Charles's wife, Camilla, Duchess of Cornwall, escorted the Obamas onto the west terrace of the palace for their formal arrival ceremony.
Mr Obama is on the second stop of a four-country European tour, arriving from Ireland late last night and planning to attend a Group of Eight summit in Deauville, France on Thursday and end the week in Poland.
While serious business awaits his talks with world leaders, he spent today getting accustomed to Buckingham Palace for only the second ever state visit by an US president to Britain. George W Bush made the first in 2003.
In the palace grounds, Mr Obama joined Prince Philip in conducting a ceremonial review of a Scots Guard regiment. Before the ceremony, the Queen and Duke of Edinburgh showed the Obamas the roughly six-room Belgian suite where they are to spend two nights. It was last used by Prince William and Kate Middleton on their wedding night last month.
The Obamas met privately with the royal newlyweds, who did not attend the arrival ceremony.
Later, after lunch at the palace, Mr Obama laid a wreath at Westminster Abbey, and held talks with prime minister David Cameron at Downing Street.
Mr Obama spent part of the afternoon playing ping pong with David Cameron and students.
"Who wants to take me on?" Obama said as he walked into the Globe Academy, a school with a high proportion of low-income students.
He and Mr Cameron played with each other against two students. At first the two left-handed leaders had some trouble getting their strategy coordinated.
"Are we keeping score?" Mr Obama asked at one point. It seemed the students got the better of the two leaders.
While the two governments have some differences over such issues as Libya, for example - where Mr Obama is seen as less eager than Europeans to lead the battle - the president and Mr Cameron stressed the unique status of US-British relations in an opinion article published in the Times of London.
"Not just special, but an essential relationship," they wrote. "It's not only history that binds us. Whether fighting wars or rebuilding the economy, our needs and beliefs are the same."
The two leaders are to announce the formation of a US-British national security council to work together on international challenges and share intelligence, an Obama administration official said.
As a precautionary measure the US president flew to London last night instead of this morning because of fears that a volcanic ash cloud from Iceland could drift over Ireland and prevent his Air Force One jet from flying.
That forced him to cut short a well-received visit to Ireland, where he sipped a pint of Guinness in the village of Moneygall to celebrate ancestral roots there and used a speech in Dublin to lift Irish spirits bruised by a severe economic downturn.
Reuters