Ludacris -- MASSIVE Brawl Erupts at Vegas Pool Party

The REAL fight in Vegas went down the day after Mayweather-Pacquiao -- a huge melee at the Rehab pool party hosted by Ludacris.

The video is absolute INSANITY -- showing multiple brawls around the pool on Sunday at the Hard Rock Hotel -- fists, chairs, bottles, ice buckets and more were hurled. Even before the fight broke out ... you can see the crowd rip a door off its hinges to burst into the party!

Jamie Foxx and Will Smith's son Trey were also at the party at some point -- though it appears they weren't involved or injured in the fight. Perks of VIP.

Here's the odd thing -- Vegas cops tell us they got no calls for this brawl. We've got a call in to the Hard Rock too.