Shortly after a beaming Harry, Queen Elizabeth's grandson, announced the birth to media, revelers began their celebrations.

"I've been sleeping out for three days ... I'm delighted they have a healthy baby and a healthy mother," said John Loughrey, a 64-year old royal fan in Windsor, west of London.

"We celebrated with them today with champagne, we'll have fish and chips and a slice of American pie!"

Britain's royal family attracts huge media interest but the younger members, especially Harry, 34, and Meghan, 37, a US-born former actress, are treated like Hollywood superstars.

Read also: Meghan, Britain's Duchess of Sussex and wife of Prince Harry, gives birth to boy

Excited tourists mixed with a handful of "superfans" clad in Union Jacks and carrying baby balloons in the street surrounding the royal family's Windsor castle estate.

Harry was present for the birth of his son, who is seventh in line to the British throne. Doria Ragland, Meghan's mother, was also with the couple at their home, Frogmore Cottage, on the estate of Windsor Castle where they married in a lavish ceremony in May last year.

"I just headed over to Buckingham Palace to see everyone else celebrating the baby," said student Nicole Wilcox.

"I was very excited, today is actually my last day in London, so the fact that she had the baby while I am still here was really exciting."

Fans and tourists gathered at Buckingham Palace whooped and cheered when, as is traditional, a framed notice of the birth was brought out and placed on a gold-colored easel in the palace forecourt.