Queen Elizabeth II, whose seven-decade reign made her the only sovereign that most Britons had ever known, died Thursday at her summer estate in Scotland, thrusting a bereaved country into a momentous transition at a time of political and economic upheaval.

The queen’s death at Balmoral Castle, announced by Buckingham Palace at 6:30 p.m., elevated her eldest son and heir, Charles, to the throne. He is Britain’s first king since 1952, taking the name King Charles III.

At 96, visibly frail, and having survived multiple health scares, the queen had been in the twilight of her reign for some years. But news of her death still landed with a thunderclap across the British realm, where the queen was a revered figure and an anchor of stability.