Charles, 73, immediately succeeded his mother but an Accession Council met at St James's - the most senior royal palace in the United Kingdom built for Henry VIII in the 1530s - to proclaim him as king on Saturday.

The council - formed of Privy Counsellors whose centuries-old role has been to advise the monarch - included his son and heir William, wife Camilla and Britain's new prime minister, Liz Truss, who signed the proclamation of his accession.

Six former prime ministers, senior bishops and a swathe of politicians cried out "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 trumpets sounded.

Soldiers in traditional scarlet uniforms shouted "hip, hip, hurrah" as White called for three cheers for the king.

Watching on were a few hundred people allowed into the court, including small children on parents' shoulders, a woman clutching flowers and elderly people on mobility scooters. Many capturing the moment on their smartphones.

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 three when she became queen in 1952.

Britain has declared a period of mourning until the state funeral for Elizabeth, which will be a public holiday.

Leaders from around the world are expected in London for the funeral, including US President Joe Biden, who said on Friday he would attend.

Charles' coronation as king will take place at a later date - and the timing for that is not yet clear. There was a 16-month gap between Elizabeth becoming queen and her coronation in 1953.

Elizabeth, who was the world's oldest and longest-serving head of state, came to the throne following the death of her father King George VI on Feb. 6, 1952, when she was just 25.

Over the decades she witnessed a seismic change in the social, political and economic structure of her nation. She won praise for modernising the monarchy during her long reign, despite intense media scrutiny and the often highly public travails of her family.

"My grandmother famously said that grief was the price we pay for love," Prince William, Charles's eldest son and now the heir, said in a statement. "All of the sadness we will feel in the coming weeks will be testament to the love we felt for our extraordinary queen."