Steve Martin Rehearses 'SNL50' Jokes at Comedy Cellar with Comedian Legends

Steve Martin is gearing up for one of the biggest performances of his career for the 'SNL' 50th anniversary special ... and he's doing it alongside some of his famous friends.

Steve turned heads on Thursday night at the legendary Comedy Cellar in NYC, when he spontaneously took the stage to workshop material. Sources with knowledge tell us his jokes are in preparation for the massive 'SNL' show this upcoming Sunday.

We're told 'SNL' legends Martin Short and John Mulaney joined Steve onstage and participated in his bit.

The night was full of comedian superstars, including surprise performances from David Spade, Chris Rock, Ray Romano, Aziz Ansari, Colin Quinn, Nate Bargatze, Leslie Jones and Marcello Hernandez. Our sources say each comedian did about 10-15 minutes of material.

Those weren't the only stars in the building ... we're told Sacha Baron Cohen, Beck, Helena Christensen and Woody Harrelson were all in the audience. In fact, there was a moment where Sacha playfully heckled comedian Jared Fried during his set.

As you can tell from our video ... the crowd was absolutely stunned by all of this. Patrons paid only $18 a ticket, expecting a normal Thursday night show, and boy did they get their money's worth!

We reached out to the owner of the Cellar, Noam Dworman, who tells TMZ ... "We've had legendary nights at the Comedy Cellar, but as a child of the seventies I've never been as starstruck as seeing Steve Martin. The term 'national treasure' is overused, but I believe Steve is one. I love 'How to Fold Soup' and everything he's ever done."