Charles III has been proclaimed as king at a ceremony at St James's Palace.
Charles became king immediately following the death of his mother Queen Elizabeth II, but a historic meeting formally confirmed his role on Saturday.
The Accession Council, a body made up of senior politicians, judges and officials, proclaimed him as the monarch in the State Apartments.
It is the first time the historic ceremony has been televised.
The King himself was not present to begin with, but he attended the second part of the ceremony to hold his first Privy Council meeting.
Clerk of the Privy Council Richard Tilbrook proclaimed Charles "King, head of the Commonwealth, defender of the faith", before declaring "God Save the King".
The packed room, including the Queen Consort, the Prince of Wales and Prime Minister Liz Truss, repeated back the phrase.
Making his own declaration, the King said: "My lords, ladies and gentlemen, it is my most sorrowful duty to announce to you the death of my beloved mother the Queen.
"I know how deeply you, the entire nation, and I think I may say the whole world, sympathise with me in the irreparable loss we've all suffered.
"It is the greatest consolation to me to know the sympathy expressed by so many to my sister and brothers.
"And that such overwhelming affection and support should be extended to our whole family in our loss."
The ceremony was attended by 200 members of the Privy Council. They included former Prime Ministers Boris Johnson, Theresa May, David Cameron, Gordon Brown, Tony Blair and John Major.
It was the decision of King Charles III to allow television cameras into the Accession Council for the first time to allow the public to view proceedings.
Following, the Garter King of Arms read the first and Principal Proclamation from the balcony above Friary Court.
Flags lowered in mourning for the late Queen will fly full-mast after the Accession Council, which is being televised for the first time.
New Zealand reading tomorrow
A wave of further proclamations will take place across the UK until Sunday, when flags will return to half-mast.
Here in New Zealand, a proclamation of accession ceremony will be held in Parliament Grounds at 12.30pm tomorrow.
Cabinet will convene beforehand for a special meeting, along with a meeting of the Executive Council, which includes the Governor-General, Dame Cindy Kiro.
It comes after the King pledged to follow his "darling mama's" life of service in an emotional first address.
He told the nation on Friday evening of his "profound sorrow" at the loss of his mother, praising her warmth, humour and "unerring ability always to see the best in people".
The King promised to serve the nation with the same "unswerving devotion" as the late Queen had during her 70-year reign.
RNZ's London correspondent Vincent McAviney earlier today spoke about what happens now.
"Tonight (New Zealand time), the pace would pick up again as proclamation events get under way," McAviney said.
"That will be televised for the first time ever in history, that is where it will officially be accepted that formerly Prince Charles is now King Charles III.
"There will then be ministerial meetings and then MPs in Parliament will swear a new oath to His Majesty, the King, and the prime minister and Cabinet will have another audience with the King at around 2.30 tomorrow afternoon."
Britain has declared a period of mourning until the state funeral for Queen Elizabeth, which will be held in a little over a week's time.
Dignitaries from around the world are expected, including US President Joe Biden.
Crowd outside palace cheers new King
On Friday, Charles was met by cheers and shouts of "God save the King!" as he met people in the crowds who had gathered at Buckingham Palace.
Later, he gave his televised address, expressing a wish that Prince William and his wife Catherine would - as the new Prince and Princess of Wales - "continue to inspire and lead our national conversation".
He expressed his "love for Harry and Meghan" and praised the "steadfast devotion to duty" of the Queen Consort.
The King acknowledged that his life had now changed, saying he would not be able to give "so much of my time and energies" to the charities and issues he had supported for decades as heir to the throne.
The 73-year-old monarch said he hoped that despite their sorrow at the Queen's death, people in the UK and the Commonwealth "remember and draw strength from the light of her example".
King Charles' statement in full
Britain's new King Charles issued a personal statement during a historic ceremony on Saturday at which he was formally proclaimed the new monarch.
Here is a text of his personal declaration:
"My Lords, Ladies, and Gentlemen.
"It is my most sorrowful duty to announce to you the death of my beloved Mother, The Queen.
"I know how deeply you, the entire Nation - and I think I may say the whole world - sympathise with me in the irreparable loss we have all suffered. It is the greatest consolation to me to know of the sympathy expressed by so many to my Sister and Brothers and that such overwhelming affection and support should be extended to our whole family in our loss.
"To all of us as a family, as to this kingdom and the wider family of nations of which it is a part, my Mother gave an example of lifelong love and of selfless service.
"My Mother's reign was unequalled in its duration, its dedication and its devotion. Even as we grieve, we give thanks for this most faithful life.
"I am deeply aware of this great inheritance and of the duties and heavy responsibilities of Sovereignty which have now passed to me.
"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.
"In this purpose, I know that I shall be upheld by the affection and loyalty of the peoples whose Sovereign I have been called upon to be, and that in the discharge of these duties I will be guided by the counsel of their elected parliaments. In all this, I am profoundly encouraged by the constant support of my beloved wife.
"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.
And in carrying out the heavy task that has been laid upon me, and to which I now dedicate what remains to me of my life, I pray for the guidance and help of Almighty God."
What happens at the Accession Council?
- BBC w/RNZ, Reuters