Those waiting in line to view the Queen lying in the state were surprised to see King Charles and Prince William.
The Prince of Wales told one young girl, “You’re over halfway,” as they praised people who had waited through the night.
William also mentioned how important it was to his family that there was a long line.
I hope you didn’t get too frozen, the King said to one in reference to the chilly overnight temps.
There were cheers of God Save the King, God Save the Prince of Wales, and hip hooray as the pair walked up and down the queue.
The queue time is now around 16.5 hours, with the line stretching five miles along the River Thames to Southwark Park.
One woman in the queue told the Prince of Wales she had been queuing for 13 hours already. He replied: “Thirteen hours? You’re looking very good on 13 hours.”
Those waiting in Lambeth, south London, told the pair the wait was “worth it” but William said he was sorry they had had to wait for so long.
The prince commented to one person waiting that their trainers were a good choice of footwear for the queue.
Several people cried after meeting Prince William, and one woman told him: “You’ll be a brilliant king one day.”
King Charles left around 20 minutes after arriving and was driven back to Buckingham Palace, but his son stayed longer to speak to more people.
The beginning of King Charles’s reign has seen the Royal Family take part in several walkabouts and meetings with the public.
On Saturday afternoon, the Earl and Countess of Wessex met crowds outside Buckingham Palace, shaking their hands and thanking them for their support.
The Queen’s youngest son could be heard asking mourners where they had come from and whether they were making their way to Green Park to lay flowers.
The pair had come from a lunch held inside the palace by the King and Queen Consort for governors-general from Commonwealth countries.
Representatives from Antigua and Barbuda, Australia, the Bahamas, Belize, Canada, Grenada, New Zealand, Papua New Guinea, Saint Lucia, Saint Vincent and the Grenadines, the Solomon Islands, and Tuvalu were among those present, along with the Prince and Princess of Wales.
On Saturday evening, the Prince of Wales, his brother the Duke of Sussex and their cousins will stand guard around the Queen’s coffin, a day after their parents held their own vigil at the Palace of Westminster.
Prince Harry has been given permission by the King to wear a military uniform.
During the vigil, Harry will stand at the foot of the coffin, with William at the head. They will be joined by their cousins’ Princesses Beatrice and Eugenie, Peter Phillips, Zara Tindall, Lady Louise Windsor, and James, Viscount Severn.
Saturday is the third full day the Queen’s coffin will lie in state in Westminster Hall, where she will remain until the morning of the funeral, on Monday.