Hundreds of people in the queue at Lambeth, south London, cheered and applauded as the King and Prince William emerged on Saturday afternoon.
The royal duo were there to thank well-wishers for paying their respects to the late monarch at Westminster Hall.
One woman offered the King condolences as he shook her hand, and another shouted: “I can’t believe this.”
Dozens shouted “hip hip hooray” as William and his father moved down the line, stopping for a few moments with each person.
Several people called their friends and relatives on the phone to tell them what they had just witnessed.
Others also shouted “God save the King” and “God save the Prince of Wales” as each passed.
William could be heard discussing how long people had waited and whether they were able to keep warm.
Several people cried after meeting him, and one woman told him: “You’ll be a brilliant King.”
The King left before William, both of them in cars surrounded by police vehicles.
It came as the queue for the Lying-In-State began to grow in Southwark Park, with a steady stream of mourners continuing to join the end of the line.
The sign at the mouth of the queue told mourners it will be a 14-hour wait minimum, which was generally greeted by relief by well-wishers, with one group cheering as they walked past.
Early on Saturday morning, the Department for Digital, Culture, Media and Sport’s queue tracker warned people not to travel to join the back of the queue.
It changed the guidance at 8am saying the line had reached Southwark Park with wait times “at least 24 hours”.
Although it was a very bright morning in central London temperatures remained cool, particularly in the shade, at around 12C.
On Saturday evening, Queen Elizabeth II’s grandchildren will stand guard around her coffin.
William will be joined by his brother the Duke of Sussex in wearing uniform at Westminster Hall.
The King received a warm welcome in London
Aaron Chown
Prince Harry, who saw action on the front line during two tours of duty in Afghanistan, has previously been denied the chance to wear his military uniform as he publicly mourns, because he is no longer a working royal.
But the King has decided his youngest son can wear uniform for the vigil, saying he will stand at the foot of the coffin, with William at the head.
William will be flanked by his cousins Zara Tindall and Peter Phillips, while Harry will be with Princesses Beatrice and Eugenie, with Lady Louise Windsor and her brother James, Viscount Severn, at the middle of the coffin.
King Charles III greeting well-wishers in the queue
Aaron Chown
Prince William speaking to mourners in the queue
Aaron Chown