Queen Elizabeth II, who ruled the UK for 70 years, passed away at Balmoral at the age of 96.
At her Scottish estate, where she had spent the majority of the summer, she passed away peacefully on Thursday afternoon.
The Queen saw significant social change after ascending to the throne in 1952.
It was a "time of tremendous anguish" for him and his family, according to her son King Charles III, and her loss will be "deeply felt" all around the world.
We deeply lament the loss of a beloved mother and beloved sovereign, he said.
The King will return to London on Friday with his wife Camilla, who is now Queen Consort, according to Buckingham Palace. He's supposed to address the country.
After the Queen's doctors expressed concern about her health earlier in the day, senior royals had assembled in Balmoral.
Following the Queen's placement under medical supervision, all of her children made their way to Balmoral, which is close to Aberdeen.
Prince William, her great-grandson and current successor to the throne, as well as Prince Harry, gathered there.
The Queen, who on Tuesday named Liz Truss as prime minister, was described as the "rock" upon which modern Britain was created and as having "given us with the stability and strength that we required."
She stated of the new King: "We offer him our loyalty and love, just as his mother so greatly and so long ago gave to so many.
And as the second Elizabethan era ends, we declare "God save the King," just as Her Majesty would have wanted, to usher in a new chapter in the glorious history of our great nation.
The Church of England's spiritual head, Justin Welby, who is also the monarch's supreme governor, expressed his "deep regret."
He remarked that "the King and the Royal Family are in my prayers."
The reign of Queen Elizabeth II as head of state covered the post-World War II period of austerity, the change from an empire to a Commonwealth, the end of the Cold War, and the UK's membership in - and exit from - the European Union.
15 prime ministers served during her time in office, including Ms. Truss, who was born 101 years after Winston Churchill, in 1874.