Hundreds of people who had been waiting for more than 10 hours to see the Queen lying in state were warmly greeted by Charles and William beneath sunny skies on Lambeth Palace Road.
The royals arrived unexpectedly at around 12.30pm, sending shockwaves through a tired and bedraggled crowd waiting to pay their respects to the Queen.
As they emerged, people surged towards the metal barriers, keen to share a few words with the King and the heir to the throne.
Catherine Padbury, 28, broke down in tears, and the Prince of Wales paused to shake her hand and check on her welfare.
Miss Padbury, who was with her parents Laraine, 59, and David, 64, all from Colchester, Essex, told the PA news agency: "I just got completely overwhelmed that they came out to speak to us because they're mourning themselves, and to come out and spend the time talking to us just means so much.
"I think he (William) saw that I was crying, paused and asked how the night was, checked that I was OK and asked who I was with."
Fighting back tears, she added: "I hope that it's some comfort to them as well, when they see how much everyone loved Her Majesty."
Emma Parrish, 28, who was with her friend Bex Mould, 34, said it was a moment they would remember "forever" and said Charles made a quip about holding them up in the queue.
Ms Parrish, an e-learning content creator from Frome in Somerset, told PA: "We were literally at the barriers.
"He (Charles) was taking his time and actually apologised for holding us up, which was really sweet - really cute and funny.
"It was just a wonderful moment. We'll remember it forever."
She praised the royals for "actually taking the time to look right into our eyes and have an actual moment with each person".
Ms Mould, a teacher from Warminster in Wiltshire, added: "We're yet to pay our respects to Queen Elizabeth II, so to have been in the presence of a past monarch, a present monarch, and then a future monarch, is quite something to behold.
"On a day like this when the sun is shining, I feel dumbfounded."
She added: "I said to Prince Charles that his mother would be proud of him, because I've been watching him ever since last Thursday and I just think he's doing miraculously whilst dealing with grief.
"He said back, 'We were lucky to have her for 70 years'."
Karen Redford, 55, a former combat medic and police officer who was wearing several medals for her service, said meeting the royals was "very emotional".
Ms Redford told PA: "The monarchy is of major importance to me, and always has been.
"I've never been this close to either of them before.
"To see Prince William that close has just blown my mind. I'm a little bit shocked."
Carol-Anne Watson, 67, a retired nurse from Peterborough, said she expressed condolences to William, and described meeting him as "overwhelming".
"No matter how long we've queued, it just gives us an amazing end to the day - and it's not over yet," she said.
Early on Saturday morning, those waiting in the queue faced temperatures of 7C, at which time the official queue tracker advised the public not to make the journey - though it didn't deter them.
