The princess, who is currently undergoing treatment for cancer, could be seen smiling and waving from the Buckingham Palace balcony at the end of a ceremony, an event filled with pomp and pageantry.

King Charles III, who is also receiving treatment for cancer, travelled in a carriage, rather than on horseback.

Thousands waited in the heavy rain for one of the biggest events in the royal calendar.

There were cheers from the crowd as they caught their first glimpse of Catherine and the King heading from Buckingham Palace to Horse Guards Parade.

The carriage procession was met by a sea of mobile phones, with people trying to catch a photograph of the royals through the windows.

The princess was pictured smiling, sitting alongside her children Prince George, Prince Louis and Princess Charlotte - with six-year-old Louis giving a wave to the crowd.

The ceremonial event marks the King's official birthday and has been held for over 260 years.

It was Catherine's first public appearance since Christmas and in March she had revealed she was undergoing chemotherapy treatment for cancer.

In a statement released on Friday, the princess said she was "not out of the woods yet", and had "good days and bad days", but was "looking forward to attending the King’s birthday parade this weekend with my family".

In a change from last year, she watched the ceremony from a balcony with her children, instead of joining other senior royals on a dais.

Louis danced to a march by the Scots Guards, and was spotted yawning while watching the parade.

His sister, Charlotte, was seen wiping steam off their carriage's windows, as they made their way back to Buckingham Palace.

The King, who also continues to have cancer treatment, travelled with Queen Camilla by carriage.

Since the general election was called last month, the palace has had to postpone some events which might have been seen as a "distraction", but the annual Trooping the Colour ceremony went ahead.

As the King's carriage emerged from behind the Buckingham Palace gates, cheers could be heard from the watching crowd.

Prince William, Princess Anne and Prince Edward rode on horseback in military uniform.

Taking the lead in this year's ceremony, as the sovereign's escort, were the Irish Guards. That role is rotated between five different regiments of Foot Guards.

During the parade, their flag was "trooped" through the ranks of soldiers.

Made up of troops from the UK, Ireland and the Commonwealth, it was described as "unique" by Sgt Michael Flood, one of the regiment's soldiers.

Meanwhile his colleague Gdsm Colati Kitione, who is from Fiji, said it was an "honour and a privilege" to be asked to take part.

The regiment's mascot - an Irish wolfhound named Seamus - also made an appearance during the parade.

As a serving Guardsman, he parades alongside a drummer, and was inspected alongside the other soldiers, before taking his place at the front of the regiment.

Back on parade today were three of the five horses of the household cavalry that bolted and ran through the streets of London in April.

One, Tennyson, featured in the Sovereign's Escort, while two others - Trojan and Vanquish - were part of the ceremonial guard for the official entrance to the palaces.

A 41-gun salute rang out from nearby Green Park, leading up to an RAF flypast over Buckingham Palace.

The Royal Family - including the King, Queen, William and Catherine - came out onto the balcony at the palace to take in the view.

The flypast finished with the Red Arrows acrobatic team flying overhead - leaving trails of red, white and blue in their wake.

Earlier in the day, the man in charge of the flypast - Air Vice-Marshall Mark Flewin - said he was "very confident" it would go ahead, despite the bad weather.

It was a grey and rain-soaked day in central London for much of this year's Trooping the Colour - with heavy showers drenching the troops and the crowds.

But that didn't stop people coming to see the annual display of pomp and pageantry first-hand - with crowds sheltering under umbrellas as they lined the Mall.

Among those watching in the stands was Prime Minister Rishi Sunak, who was spotted taking a photograph of the parade on his phone.

Carol and Brian Haddow from Scotland caught a glimpse of Kate and the Royal Family on the Mall. Mrs Haddow said like “anybody going through chemotherapy, it's a big thing for her to come out and do this”, adding that the princess seemed “fine, happy to be involved”.

One of the soldiers who helped plan Saturday's event praised Kate, saying that it was “incredibly special” for the princess to attend.

Garrison Sergeant Major Stokes told the PA news agency: “It is probably the only day we see the entire Royal Family together in public and I think the Royal Family really appreciate the day as well”.

Anti-monarchy protesters from the group Republic were among those watching on, waving big yellow flags with slogans including "Not My King".

They were a visible presence gathered on the Mall, not far from Buckingham Palace.

They had not been allowed to use amplified sound, but their loud protest chants could be heard as the royal carriages went past.

Original Story By | Sean Coughlan,