Paul McCartney and Steven Van Zandt in 2012 (Christopher Polk/WireImage)Fans who came out to see E Street Band guitarist “Little Steven” Van Zandt‘s concert with his side group The Disciples of Soul this past Saturday in London were treated to a surprise appearance by arguably the world’s most famous rock star — Sir Paul McCartney!

At Van Zandt’s show, at the London club The Roundhouse, McCartney hit the stage with guitar in hand to perform a rousing version of the classic early Beatles hit “I Saw Her Standing There.” In introducing the Fab Four legend, Little Steven told the crowd, “I got a friend of mine [who] wants to come and we need to finish some unfinished business, all right?” Van Zandt was referencing Sir Paul’s guest performance at Bruce Springsteen and the E Street Band’s set at the 2012 Hard Rock Calling festival, during which the power was cut off because of curfew restrictions.

You can watch high-quality video of McCartney rocking out with Van Zandt and the Disciples of Soul over the weekend at Vimeo.com.

Little Steven and his group’s London gig was the first show of a European tour in support of his recent album Soulfire that runs through a December 13 concert in Rome.

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Little Steven w/Paul McCartney – “I Saw Her Standing There” @The Roundhouse – London from Paul O on Vimeo.