The address was broadcast on television and streamed at St Paul's Cathedral, where 2,000 people attended a service of remembrance for Queen Elizabeth II.
In his first speech as King, Charles III said he felt “profound sorrow” over the death of his mother, Queen Elizabeth II, and vowed to carry on her “lifelong service” to the nation.
Charles made his first address to the nation as monarch on Friday evening. He became king on Thursday after the Queen’s death at Balmoral Castle in Scotland at the age of 96.
Delivering the address with a framed photo of the Queen on a desk, Charles said he planned to continue his mother's legacy with a "promise of lifelong service I renew to all today," stating that he and the rest of the Royal Family "owed her the most heartfelt debt" as sovereign and parent.
Charles also announced that his son and heir to the throne, Prince William, will succeed him as Duke of Cornwall — and as Prince of Wales.
"Today, I am proud to create [William] 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."
Charles also mentioned his younger son, Harry, expressing "my love for Harry and Meghan as they continue to build their lives overseas”.
In a heartfelt goodbye to Elizabeth II, Charles addressed her as "darling Mama", thanking her “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 concluded.
His speech was broadcast on television and streamed at St Paul’s Cathedral, where some 2,000 people were attending a service of remembrance for the queen.
Mourners at the service included newly-appointed Prime Minister Liz Truss and members of her government.