Prince William and Kate on the balcony of Buckingham Palace with their children on Saturday. AP

The annual event was also a show of stability by the monarchy after a testing few months in which both the king and Kate, wife of heir to the throne Prince William, have been sidelined by cancer treatment.

In a symbolic display of unity, Charles, Queen Camilla, William, Kate and their children were joined by other members of the royal family on a Buckingham Palace balcony at the end of the King’s Birthday Parade.

The family waved to the gathered crowd as they watched a flyby of military aircraft to cap ceremonies marking the monarch’s official birthday.

It was Kate's first appearance at a public event since December. She disclosed in March that she was undergoing chemotherapy for an unspecified form of cancer.

"I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days,” Kate said in a statement released on Friday, adding that she faces "a few more months” of treatment.

Kate said she is "not out of the woods yet” and officials stress that Saturday’s engagement doesn't herald a full return to public life.

Huge crowds turn out each June to watch the birthday parade, also known as Trooping the Color, which begins with a procession involving horses, musicians and hundreds of soldiers in ceremonial uniform from Buckingham Palace.

The 42-year-old princess traveled in a horse-drawn carriage from the palace down the grand avenue known as the Mall with her children George, 10, Charlotte, 9, and 6-year-old Louis. Bystanders cheered as they caught a glimpse of Kate, dressed in a white dress by designer Jenny Packham and wide-brimmed Philip Treacy hat.

She watched the ceremony with the children from the window of a building overlooking Horse Guards Parade, a ceremonial parade ground in central London. Louis yawned broadly at one point in proceedings, but mostly watched intently, even dancing along to the military music.

William, in military dress uniform, rode on horseback for the ceremony, in which troops parade past the king with their regimental flag, or "color.” The display of precision marching and martial music stems from the days when a regiment’s flag was an essential rallying point in the fog of battle.

Charles, who also is being treated for an undisclosed form of cancer, traveled in a carriage with Queen Camilla, rather than on horseback as he did last year.

Associated Press