King Charles III (centre) with Queen Camilla (third right) joined by Prince George (left) Prince William, Prince Louis Princess Charlotte, Kate Princess of Wales, Sophie Duchess of Edinburgh (second right) and Prince Edward on the Balcony at Buckingham Palace in London on Saturday. AP

Kate, as she is widely known, travelled in a state carriage alongside her three children at the outset of the annual event before watching the ceremonial proceedings from a viewing point.

The 42-year-old future queen then appeared on the Buckingham Palace balcony, to cheers from the crowds below who had braved torrential rain to turn out for the pageantry.

It comes nearly three months after the princess revealed she was receiving chemotherapy treatment. She had not been seen at a public engagement since a Christmas Day service last year.

In a Friday evening statement Kate said she was "making good progress" with her treatment, which is set to last for several more months, but was "not out of the woods yet".

"I'm looking forward to attending the King's Birthday Parade this weekend with my family and hope to join a few public engagements over the summer," the princess added.

Kate's cancer announcement came just weeks after it was disclosed that her father-in-law and British head of state Charles had also been diagnosed with the condition.

Neither has revealed what type of cancer they have.

'Our future queen'

Charles, 75, was given the green light to resume public duties in April, after doctors said they were "very encouraged" by his progress.

His first engagement was meeting staff and patients at a London cancer treatment centre.

Earlier this month, he attended commemoration events in northern France for the 80th anniversary of D-Day.

However, unlike previous years when he inspected troops on horseback at Trooping the Colour, Charles participated this year from a carriage, in full military regalia alongside Queen Camilla.

His elder son and heir William, 41, rode on horseback, also in military uniform.

Kate, wearing a white dress and hat, was seen arriving by car at the palace with William and their children -- Prince George, aged 10, Princess Charlotte, nine, and six-year-old Prince Louis -- ahead of the parade.

Proceedings then culminated several hours later in a fly-past by the Royal Air Force, featuring various aircraft including its aerobatic Red Arrows display team trailing red, white and blue vapours.

Senior royals looked on from the balcony, with Kate smiling at points as she stood alongside other family members.

Spectators welcomed her tentative return to public appearances.

"I was so pleased to hear the news last night," Angela Perry, a teacher in her 50s from Reading, told AFP.

"She's our future queen. She's so important," she added, calling Kate's reemergence "reassuring".

Royal officials are keen to manage expectations about Kate's gradual return to the public eye, and have maintained that her appearances will depend on her treatment and recovery.

Kate explained in her statement that she had "good days and bad days" and was "taking each day as it comes".

Protests

Trooping the Colour marks the British sovereign's official birthday and is a minutely choreographed military tradition dating back more than two centuries.

It starts at Buckingham Palace and moves down The Mall to Horse Guards Parade, where Charles received a royal salute before inspecting soldiers.

Charles was actually born in November but the second birthday tradition dates back to King George II in 1748, who wanted to have a celebration in better weather as his own birthday was in October.

However, Saturday's ceremony saw heavy downpours.

The event has its origins in the preparations for war, where all regimental flags -- or colours -- were shown to the soldiers so that they would recognise them in the confusion of battle.

This year's parade included three of five military horses that bolted through the streets of central London in April after being spooked by the noise of building construction.

London's Metropolitan Police mounted a "significant" security operation and liaised with anti-monarchy group Republic, which staged a protest on The Mall.

The force said it had banned "amplified sound" in and around the parade route on public safety grounds and to avoid disruption to the mounted regiments taking part.

Republic's activists chanted slogans and held aloft placards bearing slogans including "not my king" and "down with the crown" as the parade passed by.

Agence France-Presse