The Princess of Wales stared down anti-monarchists as they booed her and her children during their Trooping the Colour appearance.

Princess Catherine looked unamused in response to jeers as she and her children George, Louis and Charlotte rode by in a carriage in the traditional annual event. 

Anti-monarchy group Republic staged a demonstration, opening umbrellas that spelled out the words 'Stop The Reign'. 

And there were chants including 'Not my King' and 'Not my Queen', as Kate and her family passed by in central London on their way to Buckingham Palace for the occasion that also marked King Charles's official birthday - although he was actually born on November 14, 1948.

The Princess of Wales appeared unimpressed by the jibes, brushed off ahead of Royal Family members congregating on the Buckingham Palace balcony – while there was also applause from crowds gathering for the festivities. 

Charles's official birthday was celebrated with a display of military pomp and pageantry on Horse Guards Parade in front of thousands of spectators.

The Prince and Princess of Wales's youngest child Louis was spotted ducking down to look through a half-open window to catch a glimpse of Trooping the Colour, and later leaning sideways to get a better view of a celebratory flypast.

The eight-year-old prince was watching events from the first-floor window of the Duke of Wellington's former office with siblings Prince George, 12, and 11-year-old Princess Charlotte.

Others there included the Duchess of Edinburgh and Vice Admiral Sir Tim Laurence.

Below in the parade ground on a royal dais were the King, Queen and Kate, who wore a striking wide-brimmed blue and white Philip Treacy hat and matching Catherine Walker outfit.

The royal women paid tribute to their regiments, with Kate sporting the brooch of the Irish Guards, of which she serves as Colonel.

Camilla is Colonel of the Grenadier Guards and wore a red Fiona Clare dress based on their uniform and a black beret with white plume by Treacy, featuring her regimental cap badge.

Trooping the Colour is a historic ceremony where colours, or regimental flags, used as rallying points in battle were 'trooped' or paraded in front of soldiers so they would be recognised.

The ceremony evolved into a celebration of the head of state's official birthday and this year the colour trooped was the King's Colour of the Grenadier Guards, presented by Charles earlier this week during a Buckingham Palace ceremony.

Stands around Horse Guards Parade were filled with about 8,000 family members of the guards and officers.

Charles inspected the troops from a carriage, passing the ranks of about 1,000 guardsmen from the Grenadier, Scots, Irish and Coldstream Guards regiments and later stood to salute as the colours were marched past him.

The Duke and Duchess of Gloucester and Duke of Kent were also gathered on the Buckingham Palace balcony for the traditional flypast of RAF aircraft.

The aerial display roared over the palace in ten waves and featured Typhoon jets, Chinook helicopters, C-17 large military transport planes and the Red Arrows trailing their trademark red, white and blue smoke, who were joined by four F-35B short take-off and vertical-landing jets.

Crowds had gathered in The Mall to see the Royal Family but the area directly in front of Buckingham Palace was kept free of spectators.

The King wore his military uniform as he travelled with Queen Camilla from Buckingham Palace to the parade grounds.

It was a family affair for the Firm as the Prince of Wales rode on horseback while wearing the Full Ceremonial Order of the Welsh Guards. 

He was accompanied by the Duke of Edinburgh and the Princess Royal as they flashed a salute on their way to Buckingham Palace.

Highlighting the strong bond between Kate and her children, they were all dressed in complementary outfits – from George and Louis’s ties that were the exact shade of Kate’s dress to Charlotte’s printed cream dress with hints of blue.

The King was protected by the Sovereign’s Escort, made up of horseback soldiers from the Household Cavalry Mounted Regiment. 

For the first time this year, the route was lined by armed forces members from all three branches.

As with the soldiers in the procession, all are trained and operational members of the British military. 

The King celebrates his real birthday in November, but marks an official one with the Trooping the Colour event in June for ceremonial reasons.