The Duke of Cambridge rode down The Mall from Buckingham Palace on a horse as he took part in a rehearsal for Trooping the Colour.
One day he will reign as King but on Saturday he was holding the reins of a distinctive mottled grey horse called Wellesley.
Dressed in a red guardsman's uniform, William rode confidently with one white-gloved hand on the reins and one at his side.
Most of the tourists and onlookers will not have recognised the Duke, whose eyes were shaded by his traditional tall hat.
William rode behind the Duke of Kent, Colonel Scots Guards.
The event, known as the Colonel's Review, takes place a week before Trooping the Colour, which marks the Queen's official birthday.
Hundreds of soldiers were out on display for the ceremonial occasion, parading along The Mall to Horse Guards Parade in bright sunshine.
William, who served in the Blues and Royals, has not ridden at Trooping the Colour before.
After about an hour at Horse Guards Parade, the procession made its way back down The Mall.
The cavalry appeared as a sea of red shimmering the heat, with William at the centre. He was wearing the same red tunic of the Irish Guards, with a blue sash, that he chose for his wedding ceremony.