The violence started late Friday night in Atlanta ... cops say MGK and his crew were in a restaurant bar when the actor named Gabriel "G-Rod" Rodriguez walked up to him with his phone out and started recording. In the video, obtained by TMZ, you see G-Rod point at MGK and say, "I'm gonna say it because it has to be said, you're a pussy for going for family!" G-Rod tells us he was especially upset because he's a husband and father of 3 kids.

G-Rod was pissed at Kelly for his beef with Eminem -- which started when Kelly said Em's then-teenaged daughter, Hailie, is hot.

In the video, MGK appears to shove G-Rod and turn the phone away. The video stops there, but G-Rod says that's when 5 of MGK's bodyguards swooped in and, as he puts it, MGK suddenly got tough and started challenging him. G-Rod claims Kelly shoved him. He says the bar manager immediately sided with the celebrity ... and kicked out G-Rod.

G-Rod says 2 hours later he was walking toward a Hampton Inn -- across the street from the bar -- when he noticed MGK and his crew hanging outside their tour bus. When they started barking at each other an Atlanta PD officer stepped in to cool things down, but G-Rod says MGK went off ... calling the cop a "pussy" and "pig."

G-Rod says the officer did nothing in response -- which pissed of G-Rod. He told us if a black or Latino person had said that to a cop ... they'd immediately be tased or worse.

G-Rod says he went into the hotel lobby and MGK continued jawing at him ... with all his bodyguards standing around him. G-Rod, who's a pretty big dude, says he told them he'd fight each of them -- one at a time -- if they wanted a fight. Instead, he says the bodyguards all jumped him at once.

According to cops, who've seen surveillance video, one guard body slammed G-Rod and then 3 others started punching and kicking him in the face. G-Rod -- who's had roles in "Ozark," "Power" and "NCIS: New Orleans" -- says MGK wasn't involved at that point, he just let his bodyguards get physical.

G-Rod tells TMZ he's already hired a lawyer and intends to file a lawsuit against MGK, however, he adds, "I will drop the lawsuit if we both sign waivers and meet in an octagon and go man-to-man without goons or weapons."

Police are investigating, and G-Rod is at an ER getting treatment for severe facial injuries.