At Westminster Hall in London, the new monarch was joined by wife Camilla, Queen Consort, as they met with 900 members of the legislative body and members of the House of Lords, who offered their condolences in the wake of Queen Elizabeth's death.
In his reply to the address, the King said he was "deeply grateful" for the outpouring of support and also pledged allegiance to the country's constitutional monarchy.
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"I cannot help but feel the weight of history, which surrounds us," King Charles, 73, said.
"While very young, her late Majesty pledged herself to serve her country and her people and to maintain the precious principles of constitutional government which lie at the heart of our nation," the new monarch said. "This vow, she kept with unsurpassed devotion. She set an example of selfless duty which with God's help and your counsels I am resolved faithfully to follow."
King Charles also appeared emotional as "God Save the King" was sung in his presence for the first time. Camilla, 75, also joined in the U.K.'s national anthem.
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King Charles' address took place in the same chamber where his mother will lie in state later this week. She died on Sept. 8 at Balmoral Castle in Scotland at age 96.
The King will now undertake a four-nation mourning tour, with Monday marking the next part of the Queen's journey to her final resting place in Windsor.
Queen Elizabeth's coffin will travel from the Palace of Holyroodhouse, which is the official residence of the monarch in Scotland, to St. Giles' Cathedral, where it will be stationed for one day of public viewing before it is flown to London.
"I am deeply grateful for the Addresses of Condolence by the House of Lords and the House of Commons, which so touchingly encompass what our late Sovereign, my beloved mother The Queen, meant to us all."
The King’s reply to the addresses of condolence at Westminster Hall:
— The Royal Family (@RoyalFamily) September 12, 2022
The national period of mourning will last until the Queen's funeral, which will take place next on Sept. 19 at Westminster Abbey in London.