The eldest son of Britain's longest-reigning monarch has taken his late mother's place on the throne.

King Charles III is the first child of Queen Elizabeth II and Prince Philip, Duke of Edinburgh. Upon the death of his mother on Sept. 8, Charles succeeded her as king of the United Kingdom and the other Commonwealth realms.

Charles, 73, was born at Buckingham Palace in London in 1948. He was just 3 years old when he became the heir apparent as his mother acceded to the throne as queen, following the death of her father, King George VI.

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In a new proclamation, President Joe Biden has ordered flags to be flown at half-staff "until sunset, on the day of interment," as a "mark of the respect for the memory of Queen Elizabeth II."

"I hereby order that the flag of the United States shall be flown at half-staff at the White House and upon all public buildings and grounds, at all military posts and naval stations, and on all naval vessels of the Federal Government in the District of Columbia and throughout the United States and its Territories and possessions until sunset, on the day of interment," the proclamation reads.

He continued, "I also direct that the flag shall be flown at half-staff for the same length of time at all United States embassies, legations, consular offices, and other facilities abroad, including all military facilities and naval vessels and stations."

-ABC News' Molly Nagle

Former presidents of the United States have released statements in reaction to the death of Queen Elizabeth II, Britain's longest-reigning monarch.

President Bill Clinton said in a statement that he and former first lady Hillary Clinton are mourning the passing of Her Majesty with the royal family and the rest of the world, describing her life as "extraordinary."

"Throughout her remarkable 70-year reign, she led Britain through great transformations with unfailing grace, dignity, and genuine care for the welfare of all its people," Bill Clinton said. "In sunshine or storm, she was a source of stability, serenity and strength."

Bill Clinton added that he and Hillary Clinton "will always be grateful" to the kindness the queen showed them during visits to Buckingham Palace in 1995 and 2000.

President George W. Bush said in a statement that he and former first lady Laura Bush were "honored" to know the queen, describing her as a "woman of great intellect, charm and wit."

"Spending time at Buckingham Palace, and having tea with Her Majesty — and her Corgis — is among our fondest memories of the presidency," George W. Bush said.

The queen was able to lead the U.K. through dark moments with "her confidence in her people and her vision for a brighter tomorrow," George W. Bush said.

"Our world benefitted from her steady resolve, and we are grateful for her decades of service as sovereign," George W. Bush said. "Americans in particular appreciate her strong and steadfast friendship."

President Barack Obama said in a statement that he and former first lady Michelle Obama were "lucky enough to come to know Her Majesty," adding that the queen "meant a great deal to" to them.

"Back when we were just beginning to navigate life as President and First Lady, she welcomed us to the world stage with open arms and extraordinary generosity," Barack Obama said. "Time and again, we were struck by her warmth, the way she put people at ease, and how she brought her considerable humor and charm to moments of great pomp and circumstance."

Barack Obama continued, "Like so many, Michelle and I are grateful to have witnessed Her Majesty’s dedicated leadership, and we are awed by her legacy of tireless, dignified public service. Our thoughts are with her family and the people of the United Kingdom at this difficult time."

President Donald Trump also released a statement following the news of the queen’s death, saying that he and former first lady Melania Trump will always cherish their time together with the queen and will "never forget Her Majesty’s generous friendship, great wisdom, and wonderful sense of humor."

"Melania and I are deeply saddened to learn of the loss of Her Majesty Queen Elizabeth II," Donald Trump said on Truth Social, his social media platform. "Together with our family and fellow Americans, we send our sincere condolences to the Royal Family and the people of the United Kingdom during this time of great sorrow and grief. Queen Elizabeth's historic and remarkable reign left a tremendous legacy of peace and prosperity for Great Britain."

The newly appointed prime minister of the United Kingdom, Liz Truss, emphasized the importance of Queen Elizabeth II’s rule while addressing the death of Britain’s longest-reigning monarch on Thursday.

"Queen Elizabeth II was the rock on which modern Britain was built," Truss said outside of 10 Downing Street on Thursday. "Our country has grown and flourished under her reign. Britain is the great country it is today because of her."

Among the accomplishments Truss highlighted were Elizabeth’s accomplishment of growing the Commonwealth from a small group of seven countries to a "family of 56 nations" as well as visiting more than 100 countries as sovereign.

The queen, who met with Truss on Tuesday at Balmoral, invited the newly elected leader of the Conservative Party, to form a new government as prime minister, an act emblematic of the queen’s devotion to duty, Truss said.

"Earlier this week, at 96, she remained determined to carry out her duties as she appointed me as her 15th prime minister," Truss said, describing her as the "the very spirit of Great Britain."

Truss also extended condolences to the now-King Charles III, calling for the country to support him in the passing of the crown.

"We offer him our loyalty, and devotion, just as his mother devoted so much to so many for so long," Truss said.