In keeping with tradition, two gun salutes were fired in London to hail the arrival of the little Prince.
Crowds gathered to witness the King's Troop Royal Horse Artillery, resplendent in full dress uniform, riding past Buckingham Palace to Green Park where they staged a magnificent 41-gun royal salute.
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They rode from Wellington Barracks into Green Park, where 71 fabulous horses pulled six 13-pounder field guns from the First World War into position for the royal salute at 2pm.
Gun salutes are fired for the birth of every Prince or Princess, no matter where their place is in the line of succession, the Ministry of Defence said. The last royal salute for a birth was for Princess Eugenie in 1990.
The guns fired blank artillery rounds at 10-second intervals until a total of 41 shots had been fired.
"The opportunity to mark the birth of the child of the Duke and Duchess of Cambridge, by firing a 41-gun royal salute, comes as a huge honour for the King's Troop Royal Horse Artillery," said Major Mark Edward, commanding officer of the King's Troop Royal Horse Artillery.
The Honourable Artillery Company, the City of London's army reserve regiment, also fired a 62-gun salute from Gun Wharf at the Tower of London at 2pm.
While a royal gun salute normally comprises 21 guns, this increases to 41 if fired from a royal park or residence.
A total of 62 rounds were fired at the Tower of London – 21 for a normal royal gun salute is increased to 41 because it's a royal residence plus there are an additional 21 rounds fired as the citizens of the City of London show their loyalty to the monarchy.
In addition to the two royal gun salutes fired from Green Park and the Tower of London, the bells of Westminster Abbey began to ring out jubilantly at 2pm in celebration of the birth of the future King.
A team of ten bell ringers, three of whom rang the bells when Prince William was born, were due to ring 5,000 changes in a specially-composed peal called Cambridge Surprise Royal.
Guardsmen at Buckingham Palace also joined in the harmony of celebratory sounds that sounded out across in city when they played Congratulations during The Changing of the Guard on Tuesday morning.
While the capital whirls with royal baby festivities, William, Kate and their newborn son remain at the Lindo Wing of St Mary's Hospital and are not expected to leave until after 6pm on Tuesday evening or Wednesday morning.