“Queen Elizabeth’s was a life well lived,” King Charles III said, as he renewed his “darling Mama’s” promise of lifelong service.
In an emotional first address to the nation, he praised her warmth, humour and ability to see the best in people.
Prince William and Catherine will become Prince and Princess of Wales, he said, as he expressed his love for his son Prince Harry and wife Meghan.
The Queen died peacefully at Balmoral in Scotland on Thursday aged 96.
The speech was broadcast as a service to remember the late Queen, attended by senior politicians and 2,000 members of the public, got under way at St Paul’s Cathedral.
It saw the first official rendition of the national anthem – God Save the King – since Charles became monarch.
In his televised address, the King, 73, said: “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.”
He announced he had made his son William the Prince of Wales, with his wife Catherine the Princess of Wales – the title last used by William’s mother Diana.
And he expressed his “love for Harry and Meghan as they continue to build their lives overseas”.
Speaking about his wife of 17 years, Camilla, 75, who becomes the Queen Consort, he said: “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.”
He acknowledged his life had now changed, saying: “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.”
Heir to the throne Prince William will now take on the King’s former Scottish titles and responsibility for the Duchy of Cornwall.
“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,” the King said.
Looking ahead to the Queen’s funeral, the King expressed his hope that despite the sorrow felt around the nation and Commonwealth people would “remember and draw strength from the light of her example”.
He concluded: “And 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’.”