The crowd outside Buckingham Palace has grown to about 1,000 people.
People outside the gates turned round to take pictures of a rainbow that appeared after a heavy downpour.
Elizabeth II, 96, was the nation’s longest-reigning monarch, serving as head of state for more than 70 years.
The Prince of Wales is now King, having acceded to the throne immediately on the death of his mother.
In a statement, 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.”
The Queen dedicated her life to her royal duty.
She was the first British monarch in history to reach her Platinum Jubilee, and has died just three months after the national celebrations in June celebrating her 70 years on the throne.
The Queen was also the longest-reigning still-serving monarch in the world.
She mourned the loss of her beloved husband of 73 years, the Duke of Edinburgh, who died aged 99 in April 2021.
A figure of stability in national life, the Queen was head of state, the armed forces, the Commonwealth and the Church of England.
5. Members of the public gather outside Buckingham Palace
BBC One has suspended its regular programming schedule following the announcement that the Queen is under medical supervision at Balmoral. Photo: Victoria Jones
6. A rainbow over Queen Victoria Memorial, as members of the public gather outside Buckingham Palace in central London.
A rainbow over Queen Victoria Memorial, as members of the public gather outside Buckingham Palace in central London. Queen Elizabeth II is under medical supervision with the royal family rushing to be by her side amid serious health fears. Photo: Victoria Jones
7. Members of the public gather outside Buckingham Palace in central London.
Members of the public gather outside Buckingham Palace in central London. Photo: Yui Mok