Dressed in military uniform, Philip and his grandson each laid a Cross of Remembrance in front of two wooden crosses from the Grave of Unknown British Soldiers from the First and Second World Wars.
After the 92-year-old Duke and 29-year-old Harry had laid their crosses, The Last Post was played, followed by a two minutes' silence before the Princes quietly visited some of the memorial's plots.
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Having been met by The Dean of Westminster, The Rector of St. Margaret’s Church and members of The Royal British Legion Poppy factory the royals met with veterans who have served in various conflicts.
Thursday's engagement carried significance for both the Princes given that they have both served in the armed forces.
During the Second World War, Prince Philip served as part of the Royal Navy while Harry has been deployed to Afghanistan twice as an Apache Helicopter Commander.
The Field of Remembrance is a memorial garden created annually by the Royal British Legion on the lawn of St Margaret's Church between Westminster Abbey and the Houses of Parliament.
The Field, now in its 85th year, remains in place for eight days following its opening on the Thursday before Remembrance Sunday.
Elsewhere in the capital, William and Kate were also showing their support for London Poppy Day.
The Duke and Duchess will meet with volunteers and military supporters at Kensington Palace.