Britain's Queen Elizabeth views the interior of the refurbished East Wing of Somerset House at King's College in London February 29, 2012. The Queen is celebrating her sixtieth anniversary as Regent in 2012. REUTERS/Eddie Mulholland/POOL (BRITAIN - Tags: ROYALS ENTERTAINMENT EDUCATION TPX IMAGES OF THE DAY)

News that the queen's health was deteriorating emerged shortly after midday on Thursday when her doctors said she was under medical supervision, prompting her family to rush to Scotland to be by her side.

Thousands gathered outside Buckingham Palace, in central London, and there was a stunned silence when the flag was lowered to half-mast. 

The queen, whose husband died last year, had been suffering from what Buckingham Palace had called "episodic mobility problems" since the end of last year, forcing her to withdraw from nearly all her public engagements.

Queen Elizabeth II, who was also the world's oldest and longest-serving head of state, came to the throne following the death of her father King George VI on Feb. 6, 1952, when she was just 25.

On Queen Elizabeth's death, her eldest son, Charles, automatically became monarch of the United Kingdom and the head of state of 14 other realms including Australia, Canada and New Zealand. He is expected to visit all the nations of the United Kingdom in the coming days.

People shelter under their umbrellas as they gather outside Buckingham Palace, following a statement from the Palace over concerns for Britain's Queen Elizabeth's health, in London, Britain September 8, 2022. (REUTERS/Toby Melville)

"The death of Her Majesty the Queen is a huge shock to the nation and to the world," Britain’s Prime Minister Liz Truss said outside her Downing Street office where the flag, like those at royal palaces and government buildings across Britain, were lowered.

"Her legacy will loom large in the pages of British history, and in the story of our world," US President Joe Biden said in a statement. He ordered flags at the White House to be flown at half-mast.