Photos: Royal Baby Watch outside hospital
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Photos:Kate Middleton's baby bump photos
Photos:Kate Middleton, Prince William visit Scotland
The boy, weighing 8 pounds and 6 ounces, was born at 4:24 p.m. in London on Monday. His name has not been released.
Both Kate Middleton and the baby are doing fine, and will stay at St. Mary's Hospital in central London overnight.
On this side of the pond, excitement over the new royal heir has inspired some creative celebrations.
Niagara Falls is lit up in blue lights on Monday, and will be on Tuesday as well.
The Niagara Parks Commission in Canada came up with the idea to create a colorful water display in honor of the royal birth.
In Chicago, the British American Business Council hosted a celebratory tea and cocktail party at the London- based Langham Hotel.
The themed drink "Next in Line" was served in honor of the new prince.
And at Rush Copley Medical Center in Aurora, every baby born on Monday was considered royalty. Newborns who share the new royal's birth date donned crocheted crowns.
"The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news," according to the palace.
A royal proclamation outside Buckingham Palace read, "Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 4:24 p.m. today."
He is the third in line to the throne, behind his father and grandfather, Prince Charles.
"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," Prince Charles said through a statement. " 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."
Crowds are growing at Buckingham Palace.
The royal birth recalled that of the baby's father, William, in 1982, at the same central London hospital. Many remember the moment when he was carried out in his mother Princess Diana's arms with proud father Prince Charles at their side.
No one can tell what political and personal changes the intervening years will bring, but the baby can be expected to become the head of state of 16 countries, including Britain, Australia and Canada, and possibly the head of the Commonwealth, which covers 54 nations.
The child will also eventually become Supreme Governor of the Church of England.
The baby represents a living link to Britain's imperial history - the infant is the great-great-great-great-great-grandchild of Queen Victoria, who ruled at the peak of British power.
Many Britons had hoped that William and Kate would start a family shortly after their gala 2011 wedding, which drew a global television audience in the hundreds of millions.
The couple waited, however, until William was nearly finished with his military work as a search and rescue helicopter pilot based at an air base in a remote island off the coast of Wales.
That allowed Kate to ease into royal life, and to become more comfortable in the spotlight, before becoming a parent. It also allowed her to play a supporting role during Queen Elizabeth II's Diamond Jubilee celebrations last summer.
The first months of her pregnancy were not easy, and she was hospitalized in early December with acute morning sickness that left her weak and dehydrated. She seemed to recover her stamina fairly quickly and made a series of public appearances until the final weeks before giving birth, drawing praise for her poise and good cheer.
The royal couple and their newborn are expected to spend much of their time in the coming years in renovated quarters at Kensington Palace, where William and Harry also spent much of their childhood.
Royal officials say Kate and William will try to give their child as normal an upbringing as possible. That may be challenging in an age when the British royals are treated as major world celebrities.
The Associated Press contributed to this report. All rights reserved.