"Queen Elizabeth 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 got under way at St Paul's Cathedral.
In the address, the King 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."
In one of his first constitutional duties as monarch, the King, 73, held an in-person audience with Prime Minister Liz Truss at Buckingham Palace on Friday afternoon.
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