Meghan Markle is attending her first official event alongside the Queen at a service to mark Commonwealth Day.
Ms Markle, who will marry Prince Harry in May, joined her fiance and other senior royals at Westminster Abbey.
The annual multi-faith event celebrates the 53 Commonwealth countries, and this year features music from former One Direction star Liam Payne.
In her message, the Queen praised the “Commonwealth connection” which makes diversity “a cause for celebration”.
Prince Charles, the Duchess of Cornwall, the Duke and Duchess of Cambridge and Prime Minister Theresa May are all at the service.
The Duke of Edinburgh, who retired from public service last year, did not attend.
In her Commonwealth Day message, the Queen, who is head of the Commonwealth, said: “Through exchanging ideas, and seeing life from other perspectives, we grow in understanding and work more collaboratively towards a common future.
“There is a very special value in the insights we gain through the Commonwealth connection; shared inheritances help us overcome difference so that diversity is a cause for celebration rather than division.”
Liam Payne, Ghanaian drum collective One-Drum and spoken word artist Jaspreet Kaur are among those due to perform.
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Politicians in attendance include Labour leader Jeremy Corbyn, Scottish First Minister Nicola Sturgeon and former Prime Minister John Major.
Other guests include Commonwealth secretary-general Baroness Scotland, chef Ainsley Harriott, religious representatives and 800 schoolchildren and young people.
Following the service, Ms Markle, 36, will join William, Kate and Harry for a private reception.
The former Suits actress will marry Harry, 33, at St George’s Chapel, Windsor Castle at midday on 19 May.
They will travel through Windsor in Berkshire in a carriage before being married by the Archbishop of Canterbury, Justin Welby.
More than 1,000 members of the public have been invited into the grounds of the castle to watch the arrival and departure of the bride.
Monday’s Commonwealth service comes ahead of the 24th Commonwealth Heads of the Government Meeting which will be held in London in April.