US singer-songwriter Katy Perry takes selfie photos with guests at Westminster Abbey in central London during the coronations of King Charles III. AFP

The monarch arrived at the Abbey in the crimson velvet Robe of State, worn by his grandfather King George VI at his 1937 coronation, a crimson coronation tunic and cream silk overshirt with royal naval trousers.

King Charles III (L) wears the St Edward's Crown on his head as Camilla wears a modified version of Queen Mary's Crown during the coronation ceremony. AFP

Camilla wore the Robe of State originally made for the late Queen Elizabeth at her 1953 coronation over an ivory gown embellished with silver and gold floral embroidery and designed by British couturier Bruce Oldfield. Both will change robes when they leave the Abbey.

The Prince and Princess of Wales, William and Catherine, both wore formal robes and mantels over their outfits, with the heir to the throne in the ceremonial dress uniform of the Welsh Guards and his wife wearing a headpiece made of silver and crystal leaves instead of a tiara.

Both she and her daughter Charlotte wore ivory silk crepe frocks adorned with embroidered rose, thistle, daffodil and shamrock motifs - in a nod to the United Kingdom's four nations, designed by Alexander McQueen, the same label Kate chose for her wedding dress in 2011.

Charles' siblings also wore traditional robes, while his nieces - Princesses Beatrice, Eugenie, Zara and Lady Louise wore dresses or coats in fuchsia, blue and floral prints respectively.

Among the celebrity guests were actress Emma Thompson, also choosing a floral motif with a red coat adorned with rose designs and U.S. singer Katy Perry, who will perform at the coronation concert in Windsor on Sunday, dressed in an eye-catching fitted lilac short-sleeved skirt suit with matching gloves and statement hat.

Veteran actresses Maggie Smith and Judi Dench, who has portrayed both Queen Elizabeth I and Queen Victoria during her career, chose different shades of blue for their outfits while Joanna Lumley opted for a white-collared navy coat with matching white hat.

Singers Nick Cave and Lionel Ritchie, who will also perform at Sunday's concert, wore three-piece black suits, while Stephen Fry added a touch of colour with a yellow waistcoat.

There were traditional outfits from across the Commonwealth and peers wore red ceremonial robes.

Block coloured outfits were also popular with U.S and French first ladies Jill Biden and Brigitte Macron dressed in powder blue and pale pink respectively while Ukraine's first lady Olena Zelenska chose a matching sage dress and coat.

Colourful and statement hats and fascinators in fuchsia, orange and red peppered the Abbey.