Catherine, Princess of Wales, on Saturday made a tentative return to public life for the first time since being diagnosed with cancer, attending a military parade in central London to mark Britain's King Charles III's official birthday.

Kate, as she is widely known, rode in a carriage alongside her three children at the outset of the annual celebration before disembarking to watch proceedings from a viewing point.

It comes nearly three months after the future queen revealed she was receiving chemotherapy treatment. The 42-year-old princess 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 said.

Kate's announcement that she had cancer came just weeks after it was disclosed that her father-in-law, King Charles III, had also been diagnosed with the condition.

Neither has revealed what type of cancer they have.

British head of state 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.

'Our future queen'

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, had been seen arriving by car at Buckingham Palace with William and their children ahead of the parade, which formally began at 11:00 am (1000 GMT).

Spectators on The Mall leading to Buckingham Palace to witness the yearly ceremonial event welcomed Kate's tentative return to public appearances.

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

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

Royal officials will be 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".

After travelling with Prince George, aged 10, Princess Charlotte, nine, and six-year-old Prince Louis in a state carriage to watch the parade from a building, the family were set to return to Buckingham Palace for a balcony appearance.

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 will receive 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.

The ceremony 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 event will include 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 said it would mount a "significant" security operation and had been liaising with anti-monarchy group Republic, which kicked off protests at the event.

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, who huddled on a section of The Mall alongside royalists, held aloft placards bearing slogans including "not my king" and "down with the crown".