The Duchess of Cambridge, her husband Prince William and their new son have spent their first day together as a family as Britain celebrates the birth of a future king.
Cheering crowds of wellwishers rushed towards the gates of Buckingham Palace as it was announced that Catherine had given birth to a healthy male heir weighing 8lbs 6oz (about 3.5kg).
The 31-year-old duchess was admitted to St Mary's Hospital in London about 6am on Monday (local time) and the baby was born at 4.24pm after 10 hours of labour. Prince William was present at the birth.
The as-yet unnamed baby is third in line to the throne and in the direct line of succession after the Queen's eldest son and heir Prince Charles, and then his eldest son William, 31. He will be known as the Prince of Cambridge and will also become head of state of 15 Commonwealth countries including New Zealand.
In a brief statement, Prince William said that he and Catherine could not be happier.
Prince Charles has described his joy at the birth of his first grandchild, while great-grandparents the Queen and Duke of Edinburgh are also said to be delighted at the news.
"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' statement said.
"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."
A formal notice of the birth was posted on the forecourt of Buckingham Palace to the cheers of thousands of spectators outside the gates. The news was, however, announced before this via press release and Twitter rather than the more old-fashioned manner. The original protocol was for an aide to first take the bulletin to Buckingham Palace for placing in public view.
British Prime Minister David Cameron on Monday declared the birth "an important moment in the life of our nation". Landmarks including the London Eye and BT Tower were lit in red, white and blue. British armed forces will mark the event on Tuesday with gun salutes at Green Park and the Tower of London, and the bells of Westminster Abbey will be rung.
Bookmakers say the favourite names for the baby are George, James and Alexander. Prince William has been on annual leave and will take two weeks of paternity leave from his job as a Royal Air Force search and rescue pilot.
Medical staff present for the birth were surgeon-gynaecologist Marcus Setchell, obstetrician Guy Thorpe-Beeston and consultant neonatologist Dr Sunit Godambe. It was in the same hospital wing that Princess Diana gave birth to William in 1982, AFP reports.
The arrival of a royal baby and a new generation for the British monarchy had been hotly anticipated ever since the popular couple married in 2011 after a decade-long romance. They have been widely credited with revitalising "The Firm", as the British royals are known, following decades of scandal and the death of Diana in a Paris car crash in 1997.
New Zealand marks birth
The New Zealand Government formally marked the birth with a 21-gun salute in Wellington at midday on Tuesday, while landmarks and buildings around the country were lit up in blue.
Prime Minister John Key said most people will be very happy for the Duke and Duchess. He said New Zealanders will fondly remember Prince William as a toddler playing on the lawn of Government House with a toy Buzzy Bee, and it would be a great pleasure to one day welcome his son to New Zealand also.
"Wonderful news, I think. New Zealanders will be celebrating along with the happy couple and it's great to see the birth of a future king of New Zealand. We wish them all the very, very best - it will be an exciting time for them."
The Governor-General, Lieutenant-General Sir Jerry Mateparae, congratulated the royal couple, saying children are the greatest taonga (treasure) and represent all our hopes and dreams for the future.
Labour Party leader David Shearer also offered his congratulations to Catherine and William.
New Zealand's official gift to the couple is a fine lace shawl, similar to the one New Zealand gave when Prince William was born. The intricate shawl was designed by Margaret Stove and the wool was spun and knit by Cynthia Read.