The celebrations for the arrival of the new prince -- whose title will be His Royal Highness Prince (the baby's name) of Cambridge -- were hard to miss in the capital.
The fountains at Trafalgar Square were dyed blue; the BT Tower, a London landmark, flashed the words "It's a boy;" and the London Eye was illuminated in patriotic red, white and blue.
Farther afield, Canada -- where the British monarch is head of state -- turned its side of the Niagara Falls blue to mark the birth, and the CN Tower in Toronto was lit up the same color.
Betting on a name
It was a long wait for the media camped outside St. Mary's Hospital, but when the news of the birth finally came, the excitement of the moment was huge.
The Sun newspaper, Britain's best-selling tabloid daily, changed its masthead Tuesday to "The Son" to mark the occasion, above a picture of the official birth announcement, while the Daily Express and Telegraph emblazoned "It's a boy" across their front pages.
Recognizing that excitement over the prince's arrival is not universal, the Guardian newspaper's website lets users switch to a royal baby-free version of the home page. The front page of UK satirical magazine Private Eye simply says: "Woman has baby."
Many bets are being placed as the wait continues for the baby's name to be announced. British bookmakers Ladbrokes have James as favorite, followed by Henry and George, Philip, Alexander and Richard.
William's name was announced a few days after birth; his brother Harry's on departure from hospital.
Some British parents have delayed naming their newborns in recent days in hopes of either copying or avoiding the royal name, he said.
The official British Monarchy Twitter feed said: "The Queen and Prince Philip are delighted at the news of the birth of The Duke and Duchess of Cambridge's baby."
Charles Spencer, brother of the late Diana, Princess of Wales, welcomed the birth of the baby. "We're all so pleased: it's wonderful news," he said in a statement.
"My father always told us how Diana was born on just such a blisteringly hot day, at Sandringham, in July 1961. It's another very happy summer's day, half a century on."
British Prime Minister David Cameron said, "It is an important moment in the life of our nation, but I suppose above all, it's a wonderful moment for a warm and loving couple who got a brand new baby boy."
Tributes around the world
The British monarch is also head of state in 15 Commonwealth countries, including Australia, Canada, New Zealand, Belize and Jamaica.
Canadian Prime Minister Stephen Harper offered his country's congratulations on the birth of a future king.
"The arrival of the newest member of the Royal Family, a future Sovereign of Canada, is a highly anticipated moment for Canadians given the special and warm relationship that we share with our Royal Family," he said in a statement.
Barack and Michelle Obama also gave their best wishes.
"The child enters the world at a time of promise and opportunity for our two nations," the U.S. president and first lady said in a statement. "Given the special relationship between us, the American people are pleased to join with the people of the United Kingdom as they celebrate the birth of the young prince."
Russia's President Vladimir Putin and Japan's Prime Minister Shinzo Abe also sent their congratulations.
On Twitter, topics related to the royal baby jumped to the top of the trending list in the United Kingdom on Monday morning. Worldwide, hashtags such as #RoyalBabyBoy and #Will & Kate were trending later Monday.
In a nod to modern times, Clarence House called for people to send news and images of any other new arrivals using the hashtag #WelcometotheWorld.