Miley Cyrus Disses Selena Gomez On Stage -- FU, Baby! [VIDEO]

Miley Cyrus used her concert stage to wage a very public war against Selena Gomez ... singing her song "FU" while holding a cardboard cutout of the "Spring Breakers" star's face.

Cyrus was performing in Milan on Sunday when she spotted someone in the crowd holding the Gomez cutout. She snatched it up and sung with it for a bit ... before winding up and tossing it away.

It's unclear what Miley's beef with Selena is ... but there was nothing subtle about her message. We do know Miley and Justin Bieber hung out a little bit last summer.