Prince William, who is to marry Kate Middleton on Friday, met a crowd of onlookers and joked that all he had to do was get his lines right.
William will wed Kate at Westminster Abbey, in what is being described as the wedding of the decade.
The Prince, who was casually dressed in a dark jumper and white shirt, made an unexpected appearance from Clarence House at 8.30 pm on Thursday, leaving royal fans hysterical, The Sun reported.
He gave a big smile as he thanked them for their support and joked, "All I've got to do is get my lines right." He was accompanied by his best man and younger brother Prince Harry.
Two women were dressed as daffodils and William gestured to their tent and asked: "Are you going to sleep here? Have you got a jacuzzi in there?"
Many screamed "Good luck!" And he said: "You're amazing. These crowds are amazing. Thank you so much."
He spent about 10 minutes with the people.