The ceremony on Saturday is a part of historic series of events to proclaim that the monarch has died and announce the new sovereign has taken their place.
As the United Kingdom and the world continue to mourn Queen Elizabeth, who passed away at the age of 96 on Thursday, her son Charles was proclaimed King by the Accession Council in the state apartments of St James’s Palace.
The historic session proclaiming Charles III as the new sovereign was followed by the reading of the Principal Proclamation from the palace's balcony, followed by fanfare and gun salutes throughout the country.
Charles, who spent seven decades as heir apparent, automatically became King after Queen Elizabeth's death, but the accession ceremony remains a key constitutional and ceremonial step in introducing the new monarch to the country.
Scores of senior politicians past and present, including Prime Minister Liz Truss and six of her predecessors, gathered in the ornate rooms at St. James's Palace for the meeting of the Council.
They met without Charles present in the chambers, officially confirming his title, King Charles III. The King joined them to make a personal declaration, vowing to follow his mother's "inspiring example" as he took on the duties of monarch.
"I am deeply aware of this great inheritance and of the duties and heavy responsibilities of sovereignty which have now passed to me," he said.
Speaking of his personal grief, he said, "I know how deeply you and the entire nation, and I think I may say the whole world, sympathize with me in this irreparable loss we have all suffered."
The new King formally approved a series of orders — including one declaring the day of his mother's funeral a bank holiday. The date of the state funeral has not been announced, but it is expected to be around 19 September.
As this was the first accession ceremony that took place since 1952, when Queen Elizabeth II took the throne, it was also the first time the event was televised.
Charles was accompanied at the ceremony by his wife Camilla, the Queen Consort, and his eldest son Prince William. William is now heir to the throne after his father granted him the title of Prince of Wales in a public address on Friday night.
The ceremony ended with a royal official publicly proclaiming King Charles III the monarch of the United Kingdom and 14 other Commonwealth realms from a balcony at the palace.
In centuries past, this would have been the first official confirmation the public had of their new sovereign.
David White, the Garter King of Arms, made the proclamation flanked by trumpeters in gold-trimmed robes before leading cheers — "hip, hip, hooray!" — for the new king.
Gun salutes rang out in Hyde Park, at the Tower of London and at military sites around the UK as he announced the news, and scarlet-robed soldiers in the palace courtyard doffed their bearskin hats in a royal salute.
Charles says goodbye to 'darling Mama' as thousands pay respects to the Queen
Two days after the 96-year-old queen died at Balmoral Castle in Scotland following an unprecedented 70 years on the throne, people still came in their thousands to pay their respects outside Buckingham Palace in London.
The scene was repeated at other royal residences across the UK and at British embassies around the world.
Britain is holding a period of mourning for the Queen, with days of carefully choreographed ceremonies marking the death of the only monarch most people have ever known.
For many Britons, her passing is a destabilising experience. It comes at a time when many Britons are facing an energy crisis, the soaring cost of living, the war in Ukraine and the fallout from Brexit.
The country has also just seen a change in its political leadership. Prime Minister Liz Truss was appointed by the Queen on Tuesday -- just two days before the monarch died.
Charles III struck a note of continuity on Friday, vowing in a televised address to carry on the Queen's "lifelong service," with his own modernizing stamp.
The new monarch looked to both the past — noting his mother's unwavering "dedication and devotion as sovereign" — and the future, seeking to strike a reassuring note of constancy while signalling that his will be a 21st-century monarchy.
He reflected on how the country had changed dramatically during the Queen's reign into a society "of many cultures and many faiths" and pledged to serve people in the UK and the 14 other countries where he is King, "whatever may be your background or beliefs."
Spending time shaking hands with some of the thousands who came to leave flowers and pay tribute to the Queen at the gates of Buckingham Palace, Charles III was greeted with shouts of "Well done, Charlie!" and "God save the King!" One woman gave him a kiss on the cheek.
In the next few days, the Queen's body will be brought from Balmoral first to Edinburgh and then to London, where she will lie in state before a funeral at Westminster Abbey.
In his speech, Charles struck a personal note, speaking of his sorrow at the loss of "my darling Mama."
"Thank you for your love and devotion to our family and to the family of nations you have served so diligently all these years," he said, ending with a quote from Shakespeare's Hamlet — "May flights of angels sing thee to thy rest.'"