Hours after arriving in London on Friday, the new monarch said: "I speak to you today with feelings of profound sorrow. Throughout her life, Her Majesty The Queen - my beloved Mother - was an inspiration and example to me and to all my family, and we owe her the most heartfelt debt any family can owe to their mother; for her love, affection, guidance, understanding and example.
WATCH: King Charles addresses the nation
The King added: "Queen Elizabeth was a life well lived; a promise with destiny kept and she is mourned most deeply in her passing. That promise of lifelong service I renew to you all today.
"Alongside the personal grief that all my family are feeling, we also share with so many of you in the United Kingdom, in all the countries where The Queen was Head of State, in the Commonwealth and across the world, a deep sense of gratitude for the more than 70 years in which my Mother, as Queen, served the people of so many nations.
The King said his mother pledged to devote her life "to the service of her peoples" on her 21st birthday in 1947, saying: "In 1947, on her 21st birthday, she pledged in a broadcast from Cape Town to the Commonwealth to devote her life, whether it be short or long, to the service of her peoples. That was more than a promise: it was a profound personal commitment which defined her whole life."
The monarch pre-recorded the lengthy speech, which was just under 1,000 words and lasted around nine-and-a-half minutes in the Blue Drawing Room of the royal residence in central London, and it was played out at 6pm on Friday.
He paid tribute to the Queen and pledged his duty to his service as the new sovereign. "She made sacrifices for duty," the King added.
The King paid tribute to his late mother
"Her dedication and devotion as Sovereign never waivered, through times of change and progress, through times of joy and celebration, and through times of sadness and loss.
"In her life of service, we saw that abiding love of tradition, together with that fearless embrace of progress, which make us great as Nations. The affection, admiration and respect she inspired became the hallmark of her reign.
"And wherever you may live in the United Kingdom, or in the Realms and territories across the world, and whatever may be your background or beliefs, I shall endeavour to serve you with loyalty, respect and love, as I have throughout my life.
"My life will of course change as I take up my new responsibilities. It will no longer be possible for me to give so much of my time and energies to the charities and issues for which I care so deeply. But I know this important work will go on in the trusted hands of others.
King Charles has addressed the nation
"This is also a time of change for my family. I count on the loving help of my darling wife, Camilla. In recognition of her own loyal public service since our marriage 17 years ago, she becomes my Queen Consort. I know she will bring to the demands of her new role the steadfast devotion to duty on which I have come to rely so much.
"As my Heir, William now assumes the Scottish titles which have meant so much to me. He succeeds me as Duke of Cornwall and takes on the responsibilities for the Duchy of Cornwall which I have undertaken for more than five decades.
"Today, I am proud to create him Prince of Wales, Tywysog Cymru, the country whose title I have been so greatly privileged to bear during so much of my life and duty.
"With Catherine beside him, our new Prince and Princess of Wales will, I know, continue to inspire and lead our national conversations, helping to bring the marginal to the centre ground where vital help can be given.
"I want also to express my love for Harry and Meghan as they continue to build their lives overseas. In a little over a week's time, we will come together as a nation, as a Commonwealth and indeed a global community, to lay my beloved mother to rest.
"In our sorrow, let us remember and draw strength from the light of her example. On behalf of all my family, I can only offer the most sincere and heartfelt thanks for your condolences and support. They mean more to me than I can ever possibly express."
He signed off his emotional speech with a tribute to his late mother and father, saying: "To my darling Mama, as you begin your last great journey to join my dear late Papa, I want simply to say this: thank you. Thank you for your love and devotion to our family and to the family of nations you have served so diligently all these years. May 'flights of Angels sing thee to thy rest'."
The King and Queen seen at Buckingham Palace on Friday
The death of the Queen has sparked an outpouring of grief and tributes from the world. Buckingham Palace announced the death of the 96-year-old monarch, who died "peacefully" on Thursday afternoon at Balmoral. "The Queen died peacefully at Balmoral this afternoon," the statement read. "The King and The Queen Consort will remain at Balmoral this evening and will return to London tomorrow."
Charles, whose wife Camilla is now Queen, said in a written statement: "The death of my beloved Mother, Her Majesty The Queen, is a moment of the greatest sadness for me and all members of my family.
"We mourn profoundly the passing of a cherished Sovereign and a much-loved Mother. I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world.
Queen Elizabeth II died on Thursday
"During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which The Queen was so widely held."
The new King and Queen - Charles and Camilla - returned to London on Friday afternoon. Gun salutes - one round for every year of the Queen's life - were fired in central London with King Charles set to hold his first audience with the Prime Minister.
The PM and senior ministers will attend a public service of remembrance at St Paul's in central London.
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