With the hotly-anticipated Olympics Opening Ceremony only moments away, heads of state have gathered in their finery at London's Buckingham Palace, with Michelle Obama and the Duchess of Cambridge among its most glamorous guests.

Just hours after she was playing tennis in a field with London schoolchildren, the First Lady had scrapped her sports kit for a demure knee-length embroidered dress to mingle with world's most notable guests.

Mrs Obama was accompanied by the US Ambassador Louis Susman at the glitzy event, which was hosted by the Queen and Prince Philip to welcome the heads of state and government ahead of the three-hour opening ceremony later this evening.

She appeared to share a joke with the Duchess of Cambridge, whom she first met during a state visit with President Obama in May last year, as they shook hands at the reception.