Tekashi 6ix9ine was savagely beaten by a group of men inside a South Florida gym ... with injuries so bad the controversial rapper was rushed to a hospital by ambulance.

6ix9ine's attorney, Lance Lazzaro, tells TMZ ... the rapper was inside the sauna Tuesday at an LA Fitness when all of the sudden he was attacked by several guys without warning.

Lazzaro says 6ix9ine tried to fight off the combatants, but there were too many of them, beating him to a pulp before fleeing. Lance notes 6ix9ine did not have security with him at the time of the attack.

Gym staff heard the commotion and immediately notified a manager. Police and EMS were then called to the scene, and he was transported to a hospital by ambulance.

We've obtained a photo of Tekashi taken from the hospital where you can see the damage to his face -- with gashes and swelling. We're told he also suffered injuries to his jaw, ribs and his back.

The next step, Lazzaro says, is he plans to call the Feds to make sure 6ix9ine gets the protection he clearly needs. Remember, he was granted early release from federal prison after cooperating with authorities to land several of his gang member cohorts behind bars.

It's currently unclear if the gym attack had anything to do with Tekashi's cooperation.