Orlando Bloom threw a punch at Justin Bieber early Wednesday morning in Ibiza ... and Justin fled the restaurant -- this according to 2 eyewitnesses.

The eyewitnesses tell TMZ ... Orlando was in Cipriani restaurant which was packed with celebs, includingParis Hilton, and Diddy.

You don't see Orlando swing, but the eyewitnesses tell us that's exactly what happened.  We're told Justin ducked the punch.  That's where the video starts and you hear Justin
scream, "What's up bitch?"

We're told when Bieber left the crowd applauded.
0729-justin-miranda-orlando-bloom-selena-akm-gsi-cbs-sub-01The backstory is almost legendary ... Justin partied with Orlando's then-wife Miranda Kerr after a 2012 Victoria's Secret fashion show in NYC and they got very  VERY close.  And in April of this year, Orlando was hanging out with Justin's on-and-off GF Selena Gomez.  So the bad blood is flowing.
update_graphic_red_bar8:40 PM PT -- Justin just lobbed the most beautiful grenade ever by simply posting (and then quickly deleting) this pic of Miranda. It's such a d*** move, but still … kinda gangsta. Advantage Bieber?