Britain's prime minister says the country and the Commonwealth will celebrate the birth of the royal baby as news of the child's arrival spread across the globe.
A smiling David Cameron declared the birth "an important moment in the life of our nation".
Speaking outside 10 Downing Street, he said: "It is wonderful news from St Mary's, Paddington and I am sure that right across the country, and indeed right across the Commonwealth, people will be celebrating and wishing the royal couple well.
"It is an important moment in the life of our nation but, I suppose, above all it is a wonderful moment for a warm and loving couple who have got a brand new baby boy.
He said it had been a remarkable few years for the royal family, with a royal wedding, a magnificent jubilee and now a royal birth.
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He said the proud nation would be celebrating with the happy couple.
Meanwhile, Oliver Hicks, 31, a close friend of Prince William, says he is thrilled to hear the "fantastic" announcement.
The adventurer said he had been hoping for news since completing the first recorded kayak crossing of the North Sea between Britain and Norway on Friday.
"It's just such fantastic news," he said.
Archbishop of Canterbury Justin Welby was one of the first to speak of his joy on Twitter.
"Delighted for the Duke and Duchess of Cambridge as they welcome their baby boy. May God bless them all with love, health and happiness," he said.
Labour Leader Ed Miliband added: "Many congratulations to the Duke and Duchess of Cambridge. I wish them and their son all happiness and good health."
Even former Spice Girl Emma Bunton expressed her excitement - echoing a sentiment felt among the hordes gathered outside Buckingham Palace.
"It's a boy
"Special times ahead for Kate and William," she said.
