By George Bowden
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Watch: King Charles's first speech in full
"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'."
This was an intensely personal speech from King Charles, full of undisguised emotion about his family at a time of mourning.
It included a headline announcement that Prince William and Kate would become Prince and Princess of Wales, handing on to the next generation.
But perhaps more movingly he paid tribute to two women at the centre of his life – his mother the Queen and his "darling wife' Camilla.
He spoke of his "profound sorrow" at the loss his mother, her "life well lived" and her "sacrifices for duty", sticking tenaciously to her sense of service through decades of huge social change.
The new King also praised the "steadfast devotion" of the new Queen, his wife Camilla, making clear the important and non-negotiable role she would play in his life.
There was also an expression of love for "Harry and Meghan as they continue to build their lives overseas".
Another clear message being sent was that Charles was here for the long haul, the throne would be a lifelong commitment for him, for all "the remaining time God grants me".
But it was an important, tone-setting speech that was absent of any pomposity, promising to serve rather than to rule. It also touched on two of his passions, Shakespeare and religion.
Although nothing is accidental in the royal world and the speech from Buckingham Palace had its own deliberate symbolism.
He spoke in a room used by his mother for Christmas messages and in front of a posy of sweet peas and rosemary meant to represent remembrance.
And at the base of the vase there were three corgis.
Earlier, in one of his first constitutional duties as monarch, the King held an in-person audience with Prime Minister Liz Truss at Buckingham Palace.
He told Ms Truss, who was appointed by his mother on Tuesday, that the Queen's death was "the moment I'd been dreading, as I know a lot of people have, but we'll try [and] keep everything going".
Shortly before, the sun shone brightly as the new King emerged with Camilla from the state limousine outside the Palace to shake hands with people who had gathered there.
With cheers and spontaneous shouts of "God save the King", the crowd swelled against barriers assembled across the entire length of the palace forecourt.
At Westminster on Friday, MPs and Lords were paying tribute to the Queen, with Ms Truss describing her as "one of the greatest leaders the world has ever known".
It followed gun salutes and church bell tributes across the country following the Queen's death.
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WATCH: A glimpse inside King Charles's first audience with PM Liz Truss
The palace has released further details of what will happen over the coming days.
The date of the Queen's funeral is yet to be revealed, but it is expected to be in the next two weeks.
Before that, her coffin will lie at rest in Edinburgh for 24 hours and will then move to London to lie in state.
While it is lying in state in Westminster Hall, members of the public will be allowed to file past and pay their respects.
There will be no physical book of condolences for members of the public to sign, but the palace has opened an online one.
King Charles earlier declared a period of mourning across the Royal Family and Household to be observed until seven days after his mother is laid to rest at Windsor.
As part of the process of proclaiming Charles as King, a meeting of the Accession Council on Saturday will be attended by the King's son, Prince William, the new Prince of Wales.
It will also be attended by invited Privy Councillors and current serving government ministers – but that could also include former ministers, prime ministers, and senior clergy.
After the meeting, the Principal Proclamation, announcing Charles as sovereign, will be read at 11:00 BST from the balcony overlooking Friary Court at St James's Palace, central London.
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By George Bowden