Charles III was formally proclaimed King at a historic ceremony at the St James Palace on Saturday.
"It is my sorrowful duty to announce to you the death of my beloved mother, the Queen. I know how deeply you sympathise with me in the irreparable loss we have all suffered," King Charles III said at the Accession Council and Principal Proclamation.
In an emotional first address on Friday, he shared his "profound sorrow" at the loss of his mother, Queen Elizabeth II, praising her warmth, humour, and "unerring ability always to see the best in people."
Charles was met by cheers and shouts of "God save the King!" as he met people in the crowds who had gathered at the Buckingham Palace.
"I take this opportunity to confirm my willingness and intention to continue the tradition of surrendering the hereditary revenues, including the crown estate, to my government for the benefit of all, in return for the sovereign grant, which supports my official duties as head of state and head of nation," he added in his declaration.
The King promised to serve the nation with the same "unswerving devotion" as the late Queen had during her 70-year reign.
The 96-year-old monarch died on Thursday at her home in Balmoral, Scotland, surrounded by members of the Royal Family.
Charles became King the moment his mother died, but the Accession Council is held soon after the death of a sovereign to make a formal proclamation of the successor. At the council, the King makes a personal declaration about the death of the Queen and make an oath to preserve the Church of Scotland.
His wife Camilla will now have the title of Queen Consort, and the King's son, William, the new Prince of Wales.
Flags lowered in mourning for the late Queen will fly full-mast after the Accession Council.
Who Will be Attending the Funeral?
Leaders and monarchs from around the world will attend the Queen’s state funeral in London, which is expected to take place around 19 September in the same church where the Queen was crowned in 1953.
Interestingly, more than 8,000 guests descended on Westminster Abbey for the Queen’s coronation in June 1953, including Sir Winston Churchill, the prime ministers of India and Pakistan, Jawaharlal Nehru and Mohammad Ali Bogra, Gen George C Marshall – advocate of the eponymous postwar European recovery plan.
President Joe Biden, who described the Queen as “a steadying presence and a source of comfort, and pride for generations of Britons,” has confirmed that he will be attending the funeral for Queen Elizabeth II.
In a statement, Biden said that the Queen "defined an era." "In a world of constant change, she was a steadying presence and a source of comfort, and pride for generations of Britons, including many who have never known their country without her," he wrote.
Turkey’s president, Recep Tayyip Erdoğan, has also signalled his intention to attend. Members of Europe’s royal families, from countries including Spain, Belgium, Norway, Denmark, Sweden and the Netherlands, will also travel to the Abbey.
Despite the strained relations between the UK and Russia, Vladimir Putin offered his condolences to King Charles.
Bob Broadhurst, a former Metropolitan police commander, said that the funeral would almost certainly entail the biggest security operation seen in the UK.