Queen Elizabeth II, Britain's longest-reigning monarch, has died aged 96.
Her Majesty served as head of state for more than 70 years and died "peacefully" on Thursday afternoon, Buckingham Palace said.
The flag at Buckingham Palace was lowered to half-mast at 18:30, with people among the crowd gathered outside the gates crying and taking pictures as a single helicopter circled in the skies above.
- Queen Elizabeth II: Obituary
- Queen Elizabeth II: The Armed Forces' Commander-in-Chief remembered
- 'Princess Auto Mechanic': The Queen's life during Second World War – childhood to the military
In a statement released by the Royal Family, the nation's new King, King Charles III, said the death of the Queen "is a moment of great sadness" for himself and the whole Royal Family.
"We mourn profoundly the passing of a cherished Sovereign and a much-loved mother," he said.
"I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world.
"During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which the Queen was so widely held."
The Royal Family's official website carried the message: "Queen Elizabeth II 1926 – 2022" along with the official statement issued by Buckingham Palace.
The Palace said: "The Queen died peacefully at Balmoral this afternoon. The King and The Queen Consort will remain at Balmoral this evening and will return to London tomorrow."
Prime Minister Liz Truss said the Queen was the "very spirit" of Great Britain, adding: "That spirit will endure".
"The death of Her Majesty the Queen is a huge shock to the nation and to the world," Ms Truss said.
"Queen Elizabeth II was the rock on which modern Britain was built. Our country has grown and flourished under her reign.
"Britain is the great country it is today because of her."
Labour leader Sir Keir Starmer described the Queen as a "remarkable sovereign", adding: "It is a deep, private loss for the Royal Family and all our thoughts are with them at this time. The nation shares in their grief.
"We will always treasure Queen Elizabeth II's life of service and devotion to our nation and the Commonwealth; our longest-serving and greatest monarch."
Defence Secretary Ben Wallace paid tribute to the Queen, tweeting: “The sad passing today of Her Majesty The Queen will be felt by every member of her Armed Forces.
"Her Majesty was more than their Commander in Chief, she was their guardian."
Officials brought a notice confirming the Queen's death to the gates of Buckingham Palace.
Watch: A notice of the Queen's death was placed on the gates of Buckingham Palace.
A large crowd gathered to read it, and Royal Parks staff began constructing pens from metal barriers to control the public.
Born in London on 21 April 1926 as Princess Elizabeth, she was never in line for the throne until her uncle's abdication 10 years later and then her father, King George VI's, subsequent death on 6 February 1952.
She was in Kenya at the time and became the first sovereign in more than 200 years to accede while abroad and was crowned Queen Elizabeth II on 2 June 1953 in Westminster Abbey.
Prior to becoming Queen, she was part of the generation that grew up during the Second World War and even served the nation as part of the Auxiliary Territorial Service.
As a result, she became the first female member of the Royal Family to join the Armed Forces in a full-time active role and the only head of state to have served during World War Two.
This bond with the military continued throughout her reign as Queen, which included holding dozens of military ranks and patronages.
She was the wife, mother and grandmother of serving Armed Forces personnel.
In February 2022, after 70 years of service, Her Majesty The Queen became the first British monarch to celebrate a Platinum Jubilee.