Sir Paul McCartney turned down a cheeky fan's request to autograph their bottom - in front of more than 50,000 fans.
The superstar was closing a huge outdoor festival, Hard Rock Calling, in central London when he spotted the invitation on a placard.
But the suggestion hit a bum note with the musician who said he normally did not read audience signs because they affected his concentration remembering songs.
Looking into the crowd, he said: "We got one here that says 'sign my butt and I'll get it tattooed'."The answer to that is no," he added.
Sir Paul was stunned by the sight of the sea of faces as he took to the stage in Hyde Park in the early evening.
"I'm just going to take a second to drink this in - this is pretty cool," he told fans.
The star dipped back into all eras of his career with Beatles songs, such as All My Loving and Long And Winding Road, and Wings tracks such as Jet and Let 'Em In. Songs from his solo years went all the way up to his recent release under the name of The Fireman.
His song My Love, inspired by first wife Linda, was dedicated to "all you lovers out there".
Sir Paul also played an unexpected rendition of the party standard Tequila - and even he seemed surprised. Midway through he announced: "I'm not sure why we're playing this song. It just felt good."
Other well-loved tracks to get an airing included Blackbird and I'm Looking Through You, which he dedicated to John Lennon, who inspired the track.