Amy Smart Movie -- Shut it Down ... We Found a Grenade!!

Amy Smart's indie movie's already blowing up -- well, almost -- since filming is on hold right now after a real GRENADE was found on set!

The bomb scare began when location scouts showed up early Friday morning in an Altadena, CA neighborhood to begin work on "Sister Cities" ... which stars Smart and Alfred Molina.

Director Sean Hanish tells us the scouts stumbled upon what looks like an old, rusted grenade ... sitting in a driveway. They immediately called on-set fire officials. Once police got there, they decided better safe than sorry -- and phoned in the bomb squad.

Right now, the set is shut down ... and the cast and crew are being kept 500 yards back, while the cops check out the device.

This is the very FIRST day of filming. Hanish says he was only about 3 shots into filming.