King Charles III has vowed to serve the people of the United Kingdom and the Commonwealth with "loyalty, respect and love" in his first address to the nation as monarch.
In a speech pre-recorded in the Blue Drawing Room at Buckingham Palace on Friday afternoon local time, King Charles also spoke of his great sadness over the death of his "darling Mama", Queen Elizabeth II.
The 73-year-old monarch confirmed that his first-born son Prince William and his wife Kate would become Prince and Princess of Wales.
He also spoke of his love for his other son Prince Harry and his wife Meghan - whose relationship with the royal family has been strained since they stopped being working members of the royal family.
"I speak to you today with feelings of profound sorrow," King Charles said.
"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.
"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."
King Charles, an environmentalist, also signalled that he would be less vocal in his activism, as monarchs in Britain's constitution are expected to remain politically neutral.
"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," he said.
"But I know this important work will go on in the trusted hands of others."
Queen Elizabeth died at Balmoral Castle on Thursday, shortly after King Charles had dashed from nearby Dumfries House in Ayr via helicopter to be by her side.
King Charles paid tribute to his mother's "sacrifices for duty", which he said started on her 21st birthday in 1947 when she pledged in a broadcast to the Commonwealth to devote her life "to the services of the peoples".
"Her dedication and devotion as sovereign never wavered, through times of change and progress, through times of joy and celebration, and through times of sadness and loss," he said.
"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, as every member of my family can testify, she combined these qualities with warmth, humour and an unerring ability always to see the best in people.
"I pay tribute to my mother's memory and I honour her life of service. I know that her death brings great sadness to so many of you and I share that sense of loss, beyond measure, with you all."
The King contained his emotions throughout the nine-minute speech, but appeared particularly sombre in his final words directed to his late mother.
"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," he said.
"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 monarch also indicated he would remain on the throne until his death.
"As the Queen herself did with such unswerving devotion, I too now solemnly pledge myself, throughout the remaining time God grants me, to uphold the constitutional principles at the heart of our nation," King Charles said.
"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."
William, King Charles's first-born son with the late Princess Diana, would no longer use the title of the Duke of Cambridge and instead take his father's Scottish titles.
"As my heir, William now assumes the Scottish titles which have meant so much to me," King Charles said.
"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."
The King also extended an olive branch to his other son Harry and his wife Meghan, whose relationship with the royal family had become fractured, but who had returned to the UK for non-royal engagements recently.
"I want also to express my love for Harry and Meghan as they continue to build their lives overseas," he said.
Earlier on Friday King Charles travelled with his wife Camilla from Balmoral Castle to Buckingham Palace in London, where he met with crowds outside the palace and observed tributes left by mourners.
He said in his speech that he could count on the love of his wife of 17 years, Camilla, the new 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," he said.
It remains to be seen how King Charles will continue his legacy on climate issues after becoming one of the most high-profile activists on protecting the planet and the environment.
At last year's COP26 climate conference in Glasgow he said the world needed a "war-like footing" to tackle the "existential threat" of climate change.
But his warnings will now be blunted as sovereign.
John Kerry, former US secretary of state and current climate envoy, told the ABC that King Charles had been a "leader" on climate issues.
"He has convened people around the world, helped to build private sector support in particularly which he has been focused on because he understands that no government has enough money to do this by themselves," he said.
"We need to have people investing in the future, and King Charles been very much committed to that effort and has had a moral impact and a practical impact."
Mr Kerry said the new King was "ahead of his time".
"I mean you go back to the '60s and he was speaking up as a teenager, and subsequent to that, on a series of issues, has been engaged all of his life," he said.
"This is a passion for him, it's in his DNA and I hope that he will be able to continue and will continue to speak out on it."