Britain’s longest reigning monarch, Elizabeth II, died Thursday and her son became King Charles III. She was four years shy of her 100th birthday.

Flags at Buckingham Palace and other royal properties, including Balmoral Castle in Scotland where she died surrounded by family, were lowered to half-staff at 6:30 p.m. local time. The BBC played the national anthem “God Save the Queen” over images of the monarch against a black background.

Crowds gathered outside of Buckingham Palace in central London, with many reportedly weeping at the news. In a statement, the new king said: “The death of my beloved mother Her Majesty the Queen, is 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,” 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.”

The queen was last seen only two days ago, when she presided over a ceremony accepting Prime Minister Johnson’s resignation and appointed Elizabeth Truss as the new head of government. Ms. Truss was the 15th prime minister appointed by Elizabeth during her 70-year reign. Winston Churchill was the first.

Elizabeth Alexandra Mary Windsor was born in London on April 21, 1926, the first child of the Duke and Duchess of York. She acceded to the throne on February 6, 1952, only because her father’s older brother, Edward VIII, abdicated it so he could marry an American divorcee, Wallis Simpson, and her father, George VI, became king.

Barely in her teens when Britain went to war with Germany in 1939, she made, only a year later at age 14, her first public broadcast, in which she tried to comfort children who had been evacuated from London to the countryside to escape German bombing.

After the war, Elizabeth married a Royal Navy officer, Philip Mountbatten, a prince of Greece and Denmark, at Westminster Abbey. The marriage would last 73 years, until 2021, when Philip died at the age of 99.

The couple had four children — King Charles in 1948, Princess Anne in 1950, Prince Andrew in 1960, and Prince Edward in 1964. All four survive her, along with eight grandchildren and 12 great-grandchildren.

It was on a trip to Africa in 1952 that the young royal received word that her father had died in his sleep at the age of 56 after years of poor health and that she had become queen.

Elizabeth’s reign saw Britain emerge from the rubble of war only to see its once-vaunted empire, at one time the world’s largest, crumble. She met with 13 American presidents going back to Truman during her tenure, and helped foster what many came to describe as a “special relationship” between Britain and its former colony.

Relations within the House of Windsor were at times strained, especially after what Elizabeth herself called the “annus horribilis” of 1992, when three of her children divorced. The death of King Charles’s then-wife, Diana, five years later in a car crash only intensified the scrutiny.

During her Golden Jubilee in 2002, she said the country could “look back with measured pride” on the history of the last 50 years.

“It has been a pretty remarkable 50 years by any standards,” she said in a speech at the time. “There have been ups and downs, but anyone who can remember what things were like after those six long years of war appreciates what immense changes have been achieved since then.”

In 2015, she overtook her great-great-grandmother to become the longest-serving monarch in British history. Queen Victoria’s reign was 63 years, seven months and two days.

The death of her husband in 2021 was said to be a harsh blow to the aging monarch, and she began pulling back from her public duties not long after spending a night in a London hospital in October of that year. Meetings with diplomats and politicians became more virtual and she began begging off traditional fixtures on the royal calendar such as Remembrance Sunday and Commonwealth Day.

At the same time, she began taking steps that indicated she knew her days were numbered. In February, she announced that King Charles’s current wife, Camilla, would be known as “Queen Consort” when he is crowned, and she asked Charles to stand in for her to read the queen’s speech to parliament.

In June, Britain celebrated her Platinum Jubilee with days of street parties and parades. The same month she became the second-longest-reigning monarch in history, behind 17th-century French King Louis XIV, who took the throne at age 4.

In a statement Thursday, Ms. Truss reached out to the new king to offer him “our loyalty and devotion, just as his mother devoted so much, to so many, for so long.

“And with the passing of the second Elizabethan age, we usher in a new era in the magnificent history of our great country, exactly as Her Majesty would have wished, by saying the words ‘God save the King,’” she said.