Catherine, Princess of Wales, arrived at Buckingham Palace for the official celebration of the King's birthday - her first public appearance since revealing her cancer diagnosis.
The event - one of the most important events in the royal calendar - is a tradition that has marked the British monarch's official birthday for more than 260 years. However, Charles' real birthday is in November.
The princess and her husband Prince William were spotted moving together towards the palace as the Trooping the Colour ceremony began.
People of all ages gathered along Commercial Street for the event. Some of the spectators wore hats and umbrellas, while others waved flags. The most patriotic sing the national anthem.
The Princess of Wales - also known as Kate - will watch the rest of the parade with her family nearby and is expected to join fellow members of the royal family on the balcony.
It's a milestone in the Princess's recovery, marking her first public appearance since Christmas - weeks before her cancer diagnosis.
On June 14, Kate gave an update on her health, saying she had made "good progress" in her recovery. But she added that she expected her treatment to continue for several more months.
King Charles III is also involved, although he is also being treated for cancer.
Trooping the Colour is a magnificent military spectacle in which 1,400 officers and soldiers, along with 400 musicians and 200 horses, march through the streets of London from Buckingham Palace to Horse Guards Parade, with crowds lining the route.
British kings and queens have doubled down on their celebrations since the 18th century, staging both a public celebration - the official birthday party - and a more private event on the actual date.
The tradition is believed to have started with the party-crazy King George II in 1748. Like Charles III, George was born in November, when British weather was often far from ideal.
After the grand procession, the parade returns to the palace, where the monarch and family members traditionally gather on the famous balcony to greet the jubilant crowds of well-wishers below.
Kate's appearance was a major topic of discussion among onlookers, who chatted and warmed their hands with bottles of tea. "We're very excited," Rayne Geldenhuys, 41, told CNN.
"We hope to go up to the balcony and see her beautiful face again ... We missed seeing her in public, so we're here mostly for her," she added.
In a slight change from last year's inaugural procession, King Charles will take part in it from a carriage rather than on horseback. He and Queen Camilla were brought in a carriage down Commercial Street along with other royals.
This year's procedure has been adapted as the 75-year-old royal continues his treatment for an unspecified form of cancer.
Doctors have been encouraged by his progress and allowed him to return to his public duties in April, but each of his engagements are reviewed and adapted as necessary.
During the events today he will review the troops sitting in Ascot's carriage alongside Queen Camilla.
Three of the British Army horses that galloped through London in April have returned to duty and are expected to take part in this weekend's birthday parade. The remaining two horses are "enjoying a summer holiday at Chilterns" and will return to duty in due course, according to the army. | BGNES
