King Charles III, in his first public address as the United Kingdom's new monarch, said he feels "'profound sorrow" at the death of the Queen and vowed to continue his mother's "lifelong service."
In a tribute lasting around nine minutes, Charles said he shares the "great sadness of so many" and a sense of loss that is "beyond measure."
Charles's recorded speech was broadcast on television and streamed at St. Paul's Cathedral in London, where about 2,000 people, including U.K. Prime Minister Liz Truss and other senior ministers, attended a remembrance service.
Charles, who became the monarch immediately upon his mother's death on Thursday, will be formally proclaimed king at a special ceremony on Saturday.
After a separate vigil in Edinburgh, the Queen's coffin will be brought to London, and she will lie in state for several days before her funeral in Westminster Abbey.
"In a little over a week's time we will come together as a nation, as a Commonwealth and indeed a global community, to lay my beloved mother to rest," the new king said in his address.
King speaks of his 'darling mama'
"In our sorrow, let us remember and draw strength from the light of her example," he said.
"On behalf of all my family, I can only offer the most sincere and heartfelt thanks for your condolences and support. They mean more to me than I can ever possibly express."
"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," the King said.
"May flights of angels sing thee to thy rest."
In his speech, Charles also bestowed on his eldest son William and daughter-in-law Catherine the titles of Prince and Princess of Wales, which he and his late wife Diana previously held.
Senior legislators on Saturday will take an oath to King Charles III as the new monarch.
Earlier Friday, Truss spoke at the start of a special session of Parliament paying tribute to the Queen. Truss called Elizabeth "the nation's greatest diplomat" and said her devotion to duty was an example to everyone.
She said Elizabeth's death has caused a "heartfelt outpouring of grief" in the U.K. and around the world.
Truss was officially appointed by the Queen on Tuesday, just two days before her death. The prime minister said at the meeting, "she generously shared with me her deep experience of government, even in those last days."
Normal business in Parliament has been suspended and legislators will spend two days offering their memories and reflections on the Queen, who died at her Balmoral residence in Scotland after seven decades on the throne.
"The British people, the Commonwealth, and all of us in this House will support him as he takes our country forward to a new era of hope and progress," said Truss.
"The Crown endures, our nation endures and in that spirit, I say, 'God Save the King.'"
Charles and his wife, Camilla, the Queen Consort, arrived in London on Friday after leaving Balmoral. He was expected to meet with the prime minister for the first time in his official role Friday afternoon as the country began its official mourning period.
He was driven to the royal residence in London in an official Rolls-Royce on Friday. A large crowd cheered as the car arrived at the Buckingham Palace gates.
He got out of the car to greet well-wishers and look at some of the huge pile of floral tributes left to honour his mother. Some called "Thank you, Charles" and "Well done, Charlie!" as he shook hands with the crowd. Several shouted "God save the King!"
Bells tolled around Britain and in London and at military sites across the United Kingdom, and special guns fired 96 shots in an elaborate, 16-minute salute marking each year of the Queen's life.
Elizabeth was Britain's longest-reigning monarch and a symbol of constancy in a turbulent era that saw the decline of the British empire and disarray in her own family.
Labour Party Leader Keir Starmer in Parliament offered condolences to the Royal Family in their time of private grief, and paraphrased the late British poet Philip Larkin.
"It feels like, we are once again in a moment in our history where, as Larkin put it, things are 'growing strange,' where everything is spinning and the nation requires a still point," said Starmer.
But, Starmer added, "the late Queen would want us to redouble our efforts, to turn our collar up and face the storm, to carry on."
'The foundation of my life'
Hundreds of people arrived through the night to leave flowers outside the gates of Buckingham Palace, the monarch's London home, or simply to pause and reflect.
Finance worker Giles Cudmore said the Queen had "just been a constant through everything, everything good and bad."
"She's just been the foundation of my life, the country," he said.
Everyday politics was put on hold, with lawmakers set to pay tribute to the monarch in Parliament over two days, starting at noon. Many sporting and cultural events were cancelled as a mark of respect, and some businesses — including Selfridges department store and the Legoland amusement park — shut their doors.
Archbishop of Canterbury Justin Welby said the Queen's death marked an "enormous shift" for Britain and the world.
"A part of our lives we've taken for granted as being permanent is no longer there," he said.
As the second Elizabethan Age came to a close Thursday, the BBC played the national anthem, God Save the Queen, over a portrait of the monarch in full regalia as her death was announced. The flag over Buckingham Palace was lowered to half-staff. And in one of the first of many shifts to come, the anthem played Friday was God Save the King.
On Tuesday, Elizabeth presided at a ceremony at Balmoral Castle to accept the resignation of Boris Johnson as prime minister and appoint Truss as his successor.
In a statement on Thursday, Johnson mourned Elizabeth as "in many ways, the finest monarch in our history," but said that in Charles she "produced an heir to her throne who will amply do justice to her legacy, and whose own sense of duty is in the best traditions of his mother and his country."
With files from CBC News and Reuters