A court official wearing a feathered bicorn hat declared Charles "our only lawful and rightful" monarch from the balcony of St James's Palace after a historic Accession Council meeting of senior royals, clergy and government.
Six former prime ministers, senior bishops and a swathe of politicians shouted "God Save The King" as the announcement was approved.
"I am deeply aware of this great inheritance and of the duties and heavy responsibilities of Sovereignty which have now passed to me," Charles said.
"In taking up these responsibilities, I shall strive to follow the inspiring example I have been set in upholding constitutional government and to seek the peace, harmony and prosperity of the peoples of these islands and of the Commonwealth realms and territories throughout the world."
Later, on the Proclamation Gallery, a balcony above Friary Court of St James's Palace, the Garter King of Arms, David White, accompanied by others in gold and red heraldic outfits read out the Principal Proclamation, as trumpeters sounded.
"Whereas it has pleased almighty God to call to his mercy, our late sovereign lady, Queen Elizabeth the Second of blessed and glorious memory, by whose decease the crown of the United Kingdom of Great Britain and Northern Ireland, is solely and rightfully come to the Prince Charles Philip Arthur George," White proclaimed.
Soldiers in traditional scarlet uniforms shouted "hip, hip, hurrah" as White called for three cheers for the king.
Charles is the 41st monarch in a line that traces its origins to the Norman King William the Conqueror who captured the English throne in 1066. Saturday's events reflected proclamations announcing new kings and queens that date back hundreds of years.
It was the first proclamation of a monarch to be televised. And for most Britons, it was the first such event in their lifetime as Elizabeth was the only monarch they have ever known. Charles himself was just 3 when she became queen in 1952.
Following the events at St James's, a military band led soldiers, heralds and men in ceremonial dress carrying standards and pikes, through the ancient City of London to the Royal Exchange, the capital's first purpose built trading centre that dates back to 1566, where the proclamation was read again.
The announcement was also set to be delivered in other capital cities of the United Kingdom - Edinburgh in Scotland, Belfast in Northern Ireland, and Cardiff in Wales.
Queen Elizabeth's body is expected to remain at Balmoral before being taken to the Palace of Holyroodhouse in Edinburgh on Sunday.
From the Scottish capital, her coffin is due to travel to London on Tuesday to lie in state for several days.
Officials expect more than one million people to file past the catafalque in Westminster Hall, the oldest part of the UK parliament complex, before the televised funeral service at Westminster Abbey opposite.
The funeral will be a public holiday in the form of a Day of National Mourning.
King Charles's coronation, an elaborate ritual steeped in tradition and history, will take place in the same historic surroundings, as it has for centuries, on a date to be fixed.