The King is taking part in the first Trooping the Colour of his reign to mark his official birthday parade.
He is the first monarch in more than 30 years to take part in the ceremony on horseback.
Parading down the Mall from Buckingham Palace, the national anthem played during the King’s arrival on Horse Guards Parade.
During the ceremony he will take the royal salute as Colonel in Chief of the Household Division’s seven regiments, as members of the Royal Family and 8,000 spectators look on.
The colour, or regimental flag, that will be trooped in front of hundreds of Guardsmen and officers will be the King’s Colour of the 1st Battalion Welsh Guards.
The Queen and the Princess of Wales and her children were taken to the ceremony in a carriage with the Duke of Edinburgh, Prince of Wales and Princess Royal also on horseback.
The young Prince Louis cheekily entertaining spectators as he held his nose and pointed to the sky during the carriage ride to the ceremony.
Trooping the Colour is a social as well as ceremonial occasion featuring around 1,500 soldiers and hundreds of guardsmen.
Following the ceremony, the royal family will head back to Buckingham Palace and gather on the balcony to watch an extended military flypast after the display on coronation day had to be scaled down due to bad weather.
Around 70 aircraft from the Royal Navy, British Army and Royal Air Force will take part – including aircraft from the Battle of Britain Memorial Flight, the C-130 Hercules on its final ceremonial flight, modern Typhoon fighter jets with a red, white and blue finale from the Red Arrows.
The sight of a monarch riding on horseback during the parade has not been seen since 1986 when the Queen rode her charger Burmese for the final time.
The horse was a gift from the Royal Canadian Mounted Police, and Charles’ horse Noble was also from the Commonwealth country’s famous Mounties.
Following the mare’s retirement, Her Majesty opted to use a carriage for Trooping the Colour rather than train another charger.