The fighting spilled into the stands at an MMA event in Florida over the weekend -- with punches and chairs thrown everywhere -- and culminated with someone firing a GUN twice!!!

... and TMZ Sports has the unbelievable footage.

Here's the deal ... the Palm Beach County Sheriff's Office is investigating the incident at the RIZE Fighting Championship event which went down on March 20 at the Bamboo Room in Lake Worth Beach.

The event was promoted as a bantamweight championship match between Alexis Fernandez and Filipe Valentim -- but there was more action in the stands than inside the cage ... when fans started to attack each other!!

Unclear how the brawl started ... but as several groups of people engaged in fighting, one man opened up a bag, pulled out a handgun and fired it into the air -- apparently hoping the gunshot would scare everyone into behaving.

But, watch the clip ... there are people watching the fights on the balcony, directly above the shooter -- and it's a miracle no one was hit!

We heard at least one shot -- but cops say at least TWO shots were fired into the air.

Former Bellator middleweight champ Hector Lombard -- who also fought in the UFC -- was in the crowd at the event and captured the shooting on video.

"This guy just shot his handgun in the middle of a melee at an amateur show," Lombard posted on IG ... "Shameful and pathetic cowardly behavior ruined a great night of fights."

When officials arrived on-scene, paramedics treated multiple individuals who were injured in the brawl portion of the incident.

So far, no arrests have been made -- but officials say they've launched an investigation in hopes of identifying the shooter and finding out why the hell he thought it would be a good idea to pop off a couple of rounds with so many people in close proximity!