US president Barack Obama rounded off his highly successful state visit to Britain by hosting a dinner in honour of the Queen.
The American leader and his wife Michelle staged the black-tie event for the monarch and the Duke of Edinburgh as a thank you for Tuesday's state banquet at Buckingham Palace.
Mr Obama and Prime Minister David Cameron will travel to France this morning for a summit of the G8 group of major economies.
More than 50 guests, including Mr Cameron and wife Samantha, Hollywood actor Tom Hanks, Oscar winner Colin Firth, England footballer David Beckham and Harry Potter author JK Rowling attended last night's dinner at Winfield House in Regent's Park, the official home of the US ambassador to Britain Louis Susman.
The Obamas waited patiently on the mansion's doorstep for the arrival of the Queen and the Duke, where both couples greeted each other warmly.
The Queen wore an Angela Kelly white evening dress with gold and silver sequins and an emerald necklace and matching earrings, while Mrs Obama was dressed in a black off-the-shoulder dress that had a tight bodice, and wore what appeared to be a diamond necklace and matching bracelet.
The two heads of state led the way through the entrance of the residence, which serves a symbolic and political role and is a place where leading figures from all walks of life are entertained by America's official representative in the UK.
Since arriving in London on Monday, Mr Obama has taken pains to reaffirm the special relationship between the UK and the US.
Yesterday, he addressed the "mother of parliaments" at the 900-year-old Westminster Hall, where he declared that both countries "stand squarely on the side of those who long to be free".
The president rejected the argument that the rise of new economic powers such as China had sidelined Europe and America, insisting that "the time for our leadership is now".
At a "pivotal moment" in history, with demands for democracy across the Arab world and an international coalition fighting oppression in Libya, Britain and America remained "indispensable to the goal of a century which is more peaceful, more prosperous and more just", he said.
Earlier, he and Mr Cameron held talks at 10 Downing Street.
In a joint press conference, the two leaders insisted there would be "no let-up" for Muammar Gaddafi until he stood aside and allowed the Libyan people to shape their own future.
They pledged to "turn up the heat" on Tripoli and other brutal authoritarian regimes, but they stressed they had "learned the lessons" from military adventures under Tony Blair and George Bush, and were taking action in Libya as part of a wide coalition operating within the terms of a clear United Nations mandate.
Mr Obama and Mr Cameron also forged a special culinary relationship yesterday, rolling up their sleeves to jointly dish out burgers and sausages at a Downing Street barbecue for military families.
The two leaders served the cooked meat at a lunch party while their wives spooned side dishes on to guests' plates.
Later, Mrs Obama visited Oxford University's Christ Church college greeting youngsters from Elizabeth Garrett Anderson school.
She first met pupils from the north London secondary school two years ago when she paid them a surprise visit during a trip to the capital with her husband.
Tuesday's activities saw the Queen declare the US Britain's "most important ally" at a lavish state banquet in Buckingham Palace.
The first day of the visit also saw Mr Obama and Mr Cameron team up to play table tennis at a south London school.
The two leaders took off their jackets for the challenge but lost to 16-year-olds Jason Do and Jamiyu Mojaji at the Globe Academy in Southwark.
Mr Obama also joined a long list of heads of state by laying a wreath at the Grave of the Unknown Warrior at Westminster Abbey.
Meanwhile, just weeks after they married at the Abbey, the new Duke and Duchess of Cambridge travelled to the palace to meet the American couple, marking Kate's first royal duty as a member of the Royal Family.
Mr Obama bade farewell to Britain as he boarded Air Force One following his highly successful state visit.
With a warm smile and a wave, he climbed the steps of his Boeing 757 as the sun shone at Stansted Airport, Essex, a stark contrast to his late-night arrival on Monday.
His plane, destined for Normandy, France, where the president will attend a G8 group summit, took off at 9.18am.
Earlier, he kissed goodbye to his wife Michelle on the asphalt.
The first lady will be travelling back to the US separately.