Green Day fans got way more than they bargained for when they showed up to the rock band's movie casting call ... 'cause TMZ has learned they walked into a full-blown free concert!

TMZ's got the deets -- hundreds of fans flocked to the Hollywood Palladium Tuesday after the casting call went out for a New Year's Eve performance scene for Green Day’s upcoming flick, "New Year’s Rev" ... and you can see from the clip, the line literally wrapped around the block.

Sources tell us the band rocked out for hours, playing all their biggest hits while cameras rolled -- meaning these lucky superfans scored a free Green Day concert!

It was a tight ship, for sure -- fans had to request tickets in advance, and the casting call came with a laundry list of forbidden items ... including backpacks, outside food, gum, chairs, and more.

The fans were encouraged to come camera-ready in punk outfits and their favorite Green Day merch, and phones were taken at the door to prevent any leaks.

The movie, produced by Live Nation Productions, is a coming-of-age story about 3 pals -- played by Mason Thames, Kyler Coffman, and Ryan Foust -- who chase their rock star dreams all the way to L.A., mistakenly thinking their band is opening for Green Day on New Year’s Eve.