Elton John & Lady Gaga -- Shut Down The Sunset Strip For Free Show

Elton John and Lady Gaga just rocked The Sunset Strip with a surprise show.

Thousands flooded the streets Saturday to see the concert after the singers announced it just hours before ... it was mainly Elton, but Gaga joined him on stage to perform "Don't Let The Sun Go Down On Me" with the Rocket Man. The event was put on by Elton's AIDS Foundation.

Elton's got a busy weekend ahead ... he'll host his annual Oscars after-party tomorrow night in West Hollywood.