London(CNN) Queen Elizabeth II, the longest-reigning British monarch whose rule spanned seven decades, died on Thursday at the age of 96, Buckingham Palace has announced.
The Queen's oldest son Charles has now become King Charles III.
"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 royal family said in a statement posted on its official Twitter account, referring to Charles as the new King for the first time.
The King said in a statement that the Queen's death was "a moment of the greatest sadness for me and all members of my family."
"We mourn profoundly the passing of a cherished Sovereign and a much-loved Mother. I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world," he said in the statement.
Crowds of mourners gathered outside Balmoral Castle and other royal residences, despite heavy downpours in parts of the UK on Thursday evening. Many brought flowers and lit candles, some looking visibly shaken by the news.
In keeping with the royal tradition, a written statement announcing the Queen's death was displayed on the gates of Buckingham Palace. In a striking moment just after the official announcement was made, the heavy rain battering London stopped and a large double rainbow appeared over the palace.
Elizabeth ascended to the throne in 1952, on the death of her father, King George VI. She oversaw the last throes of the British empire, weathered global upheaval and domestic scandal, and dramatically modernized the monarchy.
She lost Prince Philip, her husband of 73 years and the longest-serving consort in British history, in April last year.
Elizabeth ruled over the United Kingdom and 14 other Commonwealth realms, and became one of the most recognizable women ever to have lived.
The Queen's four children were at Balmoral Castle when the announcement was made.
Charles rushed to the Scottish castle earlier on Thursday together with his wife, Camilla. The Queen's daughter, Princess Anne, known as the Princess Royal, was already there.
Prince William, who is now the heir apparent to the throne, arrived at Balmoral Thursday afternoon together with the Queen's other two sons, Prince Andrew and Prince Edward, as well as Edward's wife Sophie, the Countess of Wessex.
Prince William and his wife Catherine have taken on the title of Duke and Duchess of Cornwall, according to their official Twitter account. Charles and Camilla were previously known by that title. The couple are now known as the Duke and Duchess of Cornwall and Cambridge.
Catherine remained at Windsor, where their three children attended their first day at a new school.
Prince Harry arrived to Balmoral after the announcement was made. His wife Meghan, Duchess of Sussex was not traveling with him.
The Queen was last seen in public on Tuesday when she formally appointed Liz Truss as the UK's new prime minister. A photograph from the audience showed the monarch smiling, standing in the drawing room in Balmoral, carrying a walking stick. Truss is the 15th -- and the last -- British Prime Minister to be appointed by Elizabeth.
There have been concerns over the Queen's health ever since a brief hospital stay last October. She has experienced episodic mobility issues, which have at times caused her to withdraw from official engagements.
But those concerns grew deeper on Wednesday when Buckingham Palace announced the Queen had postponed a virtual meeting of her Privy Council after being advised by doctors to rest.
On Thursday, the palace announced that the Queen was under medical supervision, but said she was "comfortable" at Balmoral. As her children rushed to her side during the day, it became clear the situation was serious.
Her death comes seven months after the Queen marked the 70th anniversary of her accession to the throne. The UK officially celebrated the platinum jubilee in June with days of pomp and pageantry and she made several public appearances in London.
Messages of condolences started pouring in from around the world immediately after the announcement was made, underscoring the global impact the Queen had made during her 70-year reign.
Speaking outside Downing Street on Thursday, Truss said the Queen's death was "a huge shock to the nation and to the world."
"Queen Elizabeth II was the rock on which modern Britain was built," the new prime minister said. "Our country has grown and flourished under her reign."
Scottish First Minister Nicola Sturgeon paid tribute to Queen Elizabeth II, saying she was "loved and admired" by the people of Scotland.
United Nations Secretary-General António Guterres described the Queen as a "good friend" of the UN, adding: "She was a reassuring presence throughout decades of sweeping change, including the decolonization of Africa and Asia and the evolution of the Commonwealth."
Pope Francis also mourned the Queen's death, praising her "steadfast witness of faith in Jesus Christ" in an open letter to King Charles III.
Canadian Prime Minister Justin Trudeau said it was "with the heaviest of hearts" that Canada learned of the monarch's passing, while Australia's PM Anthony Albanese praised the Queen's devotion to "duty, family, faith and service."
New Zealand Prime Minister Jacinda Ardern said in a national address that the Queen had come to define "notions of service, charity, and consistency."
The Queen was the head of state in Canada, Australia and New Zealand.
US President Joe Biden and first lady Jill Biden called the Queen a "steadying presence."
"Her Majesty Queen Elizabeth II was more than a monarch. She defined an era," they said in a statement.
President of Ireland Michael D. Higgins said the country had lost "a remarkable friend."
"Her Majesty served the British people with exceptional dignity. Her personal commitment to her role and extraordinary sense of duty were the hallmarks of her period as Queen, which will hold a unique place in British history," Higgins said. In a hugely symbolic moment in 2011, the Queen became the first British monarch to make a state visit to the Republic of Ireland.
Numerous kings, queens and royal families of other countries have also sent messages of sympathy. King Felipe VI of Spain, King Abdullah II of Jordan, Norway's King Harald and King Willem-Alexander, Queen Máxima and Princess Beatrix of the Netherlands all sent condolences.
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