King Charles III along with other members of the British royal family at the balcony of Buckingham Palace after the birthday parade on Saturday. AFP

Britain's royal family rounded off King Charles III's inaugural birthday parade on Saturday with a Balcony appearance at Buckingham Palace to watch a spectacular fly-past.

Three of the king's young grandchildren -- future king Prince George, nine, Prince Louis, five, and Princess Charlotte, eight -- joined the rest of the family on the Balcony with the princes sporting red ties and blue blazers and Charlotte in a sailor suit with red trim.

They were cheered by the crowds who gathered outside the palace and in The Mall, the avenue leading up to it.

The air display of some 70 military aircraft, following a 41-gun salute from nearby Green Park in central London, came after bad weather cut short a planned fly-past at Charles's coronation on May 6.

It ended with the Red Arrows, the Royal Air Force's aerobatic display team, trailing red, white and blue vapours.

Earlier, Charles saddled up for the annual Trooping the Colour parade that marks the British sovereign's official birthday.

It was the first time the monarch has ridden at the ceremony since his mother, Queen Elizabeth II, in 1986.

Charles, who also took the royal salute, was followed on horseback as he inspected the troops by his eldest son and heir, Prince William, Charles' brother Edward, the Duke of Edinburgh, and sister Anne, the Princess Royal.

Queen Camilla in a military-inspired red outfit, and William's wife Catherine, the Princess of Wales, who was dressed in green, followed in a carriage.

The colourful display of regimental precision and pageantry was the first of 74-year-old Charles's reign.

Charles's actual birthday is on November 14 but British sovereigns celebrate twice -- once in private and again in public.

Colourful display

The June parade tradition began in 1748 under King George II, who wanted a celebration in better summer weather, as his own birthday was on October 30.

The televised event kicked off with a procession from Buckingham Palace to Horse Guards Parade.

Some 1,400 soldiers, 400 musicians and 200 horses took part, led in the parade by Juno, a 10-year-old shire mare, alongside three other Drum Horses -- Perseus, Atlas and Apollo.

Drum Horses are the most senior animals in the army and hold the rank of major. They are traditionally named after figures from Greek mythology.

The minutely choreographed event has its origins in the display of colours or flags of different regiments to allow their soldiers to identify them in battle.

The 1st Battalion Welsh Guards trooped their colour up and down the ranks this year.

The UK is currently experiencing a hot spell which would have made it difficult for the troops in their ceremonial black bearskin hats and thick red tunics.

Unlike last weekend, however, when William inspected troops from the Household Division group of senior regiments, none of the soldiers appeared to faint.

As Prince of Wales, William is honorary colonel of the Welsh Guards.

Charles -- who as head of state is commander-in-chief of the armed forces -- later led the soldiers back to the palace.

Queen Elizabeth last rode her horse, Burmese, a gift from the Royal Canadian Mounted Police, at the parade in the mid-1980s.

After the horse -- which she rode for 18 years -- was retired in 1986, she decided to use a carriage for Trooping the Colour.

Last year's parade was the last for the late queen and formed part of four days of events to mark her record-breaking 70th year on the throne.

It was one of her final public appearances before her death, aged 96, in September.