The Queen, 78, is scheduled to accompany the King for a busy programme of royal events during Trump’s state visit on Wednesday and Thursday.
The Prince and Princess of Wales, William and his wife Catherine, also joined the mourners at Westminster Cathedral.
Prince Andrew, the Duke of York, and his ex-wife Sarah, the Duchess of York, were also seen in attendance.
The Duchess of Kent, who died on September 5 at the age of 92, was known for her links to the Wimbledon tennis tournament and for anonymously teaching music at a primary school.
Pallbearers from the Royal Dragoon Guards carry Katharine, Duchess of Kent’s coffin. Photo / Getty Images
A talented pianist, organist and singer, she was born Katharine Worsley into an aristocratic family in Yorkshire, northern England.
In 1961, she married Prince Edward, Duke of Kent, who is a first cousin of the late Queen Elizabeth and remains, at 89, a working member of the royal family.
Camilla is due to receive the Donald and Melania Trump alongside the King at Windsor Castle on Wednesday, around noon local time.
– Agence France-Presse