Hundreds flocked to the private Lindo Wing at St. Mary's Hospital in Paddington, London and gathered outside Buckingham Palace when the news broke just before 9pm that Kate had safely delivered a baby boy - the third-in-line to the throne - at 4.24pm that afternoon.
The child, who will be known as the Prince of Cambridge, has not been named yet. He weighed in at 8lbs 6oz.
In a very brief statement, the proud new dad said that he and Kate "could not be happier".
Cheers were heard outside the hospital when news of the royal birth filtered through the tourists and reporters who had been camped outside on the hottest day of the year so far. Breaking with tradition, it was announced via press release before a royal aide took the official birth notice to Buckingham Palace.
Outside the Lindo Wing, a town crier also announced the birth by reading from a scroll and ringing a bell, adding to the atmosphere of jubilation sweeping the capital.
Over at Buckingham Palace, tourists and well-wishers streamed in to get a look at the official birth announcement that was posted on the forecourt of the palace. Well-wishers outside the hospital saw press secretary Ed Perkins leave the Lindo Wing with the notice enclosed in a folder. He handed it to a colleague in a car, who then took the notice to the palace.
The announcement stated the gender and time of the birth of the royal arrival, and was signed by the doctors who treated Kate.
So many people arrived that one crowd member interviewed by BBC joked it was just as difficult trying to get away from the birth announcement as it was trying to see it.
Elsewhere, the BT Tower in central London was illuminated blue, as the words 'IT'S A BOY' flashed across its highest point.
The London Eye also got into Royal Baby spirit by lighting up in red, white and blue once the happy news was confirmed.
Many took to the streets wrapped in Union Jack flags to share their excitement at the arrival of the third-in-line to the throne, while one taxi driver drove around with an 'It's a boy' sign on his roof.
Today, the bells of Westminster Abbey will ring to mark the new arrival and two separate gun salutes will take place in the capital.
Meanwhile, several statements have come from members of the royal family, most notably Prince Charles and The Queen.
Prince William's father Prince Charles issued a statement this evening on behalf of himself and Camilla, Duchess of Cornwall saying: "Both my wife and I are overjoyed at the arrival of my first grandchild. It is an incredibly special moment for William and Catherine and we are so thrilled for them on the birth of their baby boy.
"Grandparenthood is a unique moment in anyone's life, as countless kind people have told me in recent months, so I am enormously proud and happy to be a grandfather for the first time and we are eagerly looking forward to seeing the baby in the near future."
The Queen and Prince Phillip have also issued a very brief statement, saying they are "delighted" by the arrival.
It's thought we will get our first glimpse at the royal child when Kate and William leave hospital and pose for a traditional picture on the front steps, although a time for this has not been confirmed yet.
Cheryl Cole, Katie Price, more stars congratulate Kate and William:
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