The Prime Minister said the "whole country will celebrate" the birth of the royal baby as news of the child's arrival spread across the globe.
David Cameron offered his congratulations on Twitter where the worlds of showbiz, politics and sport appeared to share in the Duke and Duchess's delight.
He wrote: "I'm delighted for the Duke and Duchess now their son has been born. The whole country will celebrate. They'll make wonderful parents."
Meanwhile, Oliver Hicks (31), a close friend of Prince William, said he was 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. "I hope I'll get to see him soon."
Archbishop of Canterbury the Most Rev 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.
Deputy Prime Minister Nick Clegg said the "good news" would make the "whole country smile" as he offered his congratulations on the micro-blogging site.
And First Minister of Wales Carwyn Jones wished the young royals the "very best" as they became parents for the first time.
"The couple already have established and strong links with Wales, choosing Anglesey to start their married life, and they will always have a very warm welcome home here as a family."
Scottish First Minister Alex Salmond added: "I am sure that people across Scotland will be absolutely thrilled to hear the news of the birth of a baby boy to the royal couple and will want to join me in wishing the proud parents many congratulations."
Mayor of London Boris Johnson said: "Congratulations to the Duke and Duchess of Cambridge on their new arrival."
Foreign Secretary William Hague added: "Wonderful news and a happy moment for our country and Overseas Territories."