Not even the drizzle that day – 21 June, 1982 – could dampen what the media described as a "carnival atmosphere".
CLICK ON PHOTOS FOR FULL GALLERY
William's parents showing him off 21 hours after his birth
In an age old custom that will be repeated for the Duke and Duchess of Cambridge's child, the news of his arrival was posted on the gates of Buckingham Palace.
Signed by Diana's doctors, the notice said that the Princess of Wales had been "safely delivered of a son and both mother and baby were doing well". The crowd responded by shouting irreverently: ""Nice one, Charlie, let's have another one."
Meanwhile, a palace spokesman informed the public that a baby boy with blue eyes, weighing 7lbs 1 ½ oz, had been born, adding: "He cried lustily".
The newborn was also toasted by British Airways passengers around the world, who enjoyed a complimentary glass of a special Royal Birth Celebration Cocktail.
Crowds including a self-appointed town crier waiting for news outside Buckingham Palace in 1982
The public didn't have to wait too long for the first sight of their little Prince; his mother left hospital just 21 hours later, cradling her bundle of joy in her arms.
The christening of William Arthur Philip Louis took place six weeks later in the Music Room of Buckingham Palace. Since World War II when the palace chapel was destroyed, nearly all royal baptisms have taken place in the Music Room or at the chapel in Windsor Castle.
Wearing the Honiton lace gown created for Queen Victoria's children and used for all Windsor children since 1841, he was christened with water from the River Jordan.
Gun salutes in honour of the future King
Support and enthusiasm for Charles and Diana was just as apparent for their second child and the press noted that the couple appeared particularly close, holding hands in the back of the car on the way to hospital. On 17 September, 1984 the Western Mail reported that an "impromptu champagne party" was held at a polo match to celebrate Prince Harry's birth.
Many years previously their father's appearance had been greeted with the fanfare expected for a future King.
Thousands were waiting outside Buckingham Palace just before midnight on 14 November 1948 when a footman appeared. He called a policeman to the gate and whispered to him. "It's a Prince," said the officer, causing the crowds to erupt in cheers and laughter.
The news was immediately cabled across the Empire and the Commonwealth, where church bells were rung and Union Jacks raised in Charles' honour. British warships fired a salute.
William entertaining the royals at his brother's christening
Gun salutes also marked his sister Princess Anne's arrival – 41 in Hyde Park and 62 at the Tower of London.
The crowd watching the fourth cricket test at the Oval were told over the loud speaker: "Ladies and gentelemen, we have got a new little Princess".
The proud father, the Duke of Edinburgh, toasted the health of the newborn with champagne while the Queen – then Princess Elizabeth – rested after the labour which had taken place on the second floor of their home Clarence House.
His father playing a similar role at Princess Anne's baptism
At Balmoral in Scotland, her father the King had prepared a batch of telegrams to flash to all Commonwealth countries, with a space left for the word 'boy' or 'girl'. As soon as Prince Philip rang from London the monarch's private secretary filled in the blank and despatched the news to Commonwealth governments.
With the prevalence of social media, it will be seconds before the birth of the Queen's great-grandchild is known around the globe. Then the only question will be about the name. When Prince Harry was born that was released immediately. With his older brother well-wishers had to wait a whole week before an announcement was made – an eternity in the Twitter age.
- Gun salutes are the firing of cannons or firearms as a mark of respect or welcome
- Custom stems from naval tradition, in which a warship would fire its cannons out to sea until all ammunition was spent, signifying peaceful intentions
- Basic number of rounds is a 21-gun salute
- Total varies for occasion and rank of person being celebrated
- In Hyde Park an extra 20 rounds is fired because it is a royal park
- At the Tower of London 62 rounds are fired on royal anniversaries (the basic 21, plus 20 because the Tower is a royal palace and fortress, plus another 21 'for the City of London') and 41 on other occasions
- Saluting stations are located in London, Edinburgh Castle, Cardiff and Hillsborough Castle in Northern Ireland
- Royal anniversaries include Coronation Day, birthdays of the Queen, Prince Philip and Prince Charles and the State Opening of Parliament