Charles, who raced to be by the side of the Queen at her Scottish home on Thursday, headed back to London with his wife Camilla, now Queen Consort, before a meeting with the prime minister and making a televised statement at 6pm.
The death of the Queen, Britain's longest-reigning monarch and a towering presence on the world stage for 70 years, has brought messages of condolences from around the world.
Outside Buckingham Palace in central London, Britain's new monarch was met with cheers, applause and a crowd singing God Save The King as he made his first public appearance since ascending the throne.
"I said thank you for coming and I'm so sorry, and he said thank you to me," said Denise Berriman, one of those the new King spoke to during his impromptu 10-minute walkabout after stopping his car outside the palace gates.
"Then I told Camilla to look after Charles, and she said 'I'll try'. I was amongst the first people to shake the hand of the King of England. I was two-and-a-half when Elizabeth became Queen, and we will never have someone like her again, and I'm 73 now watching her son become king."
Charles and Camilla then briefly inspected the mass of flowers left outside the famous black railings, before heading into the palace where the flag of the British sovereign was flown overhead.
The United Kingdom had woken up to its first day without a woman once described by her grandson Harry as "the nation's grandmother".Â
Billboards across the city displayed messages of condolence and newspapers ran front-page photo tributes to the queen.
Buckingham Palace said there would be a period of mourning to be observed by members of the family and the royal household until a week after the funeral, the date of which has not yet been confirmed but is expected in about 10 days time.
Charles will officially be proclaimed king on Saturday at a meeting of the Accession Council held at St James's Palace followed by proclamations across the the UK.
