The proclamation has been read out by the Accession Council and Prince Charles Philip Arthur George is now Charles III.
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 new King announced the death of his “beloved mother, the Queen”, adding that “The whole world sympathises with me in the irreparable loss we’ve all suffered.”
King Charles III pledged to uphold the constitution with the help of God Almighty just as his mother did.
“My mother’s reign was unequalled in its duration, dedication and 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,” he said.
Charles III then took the oath related to the security of the Church of Scotland and then signed the oath he has just declared in front of the Privy Council.
Witnesses including Prince William – the new Prince of Wales, Camilla, the Queen Consort have added their signatures to the document.
Prime Minister Liz Truss and Archbishop Justin Welby watched on as the signing of the proclamation took place.
Former UK prime ministers – including Gordon Brown, David Cameron Boris Johnson and Theresa May – lined up as the proclamation of the new king is read out.
Councillors lined up in the Throne Room of St James’s Palace for part two of the ceremony to greet the King.
Currently, members are exiting the Throne Room and signing the proclamation.
Also, the new King is about to be proclaimed publicly on the balcony overlooking Friars’ Court.