This wild video is making the rounds online -- and it captures the moment in a Spanish movie theater many are calling a real "cat fight" ... kicking off when a man in green athletic pants makes his way toward his waiting adversary at the bottom of the stairs.

People are trying to hold the guy back, but he gets through and the two dudes start going at it -- and the man who called out his opponent in green probably regrets it pretty quick.

The fighter lands a flurry of punches and kicks before the other man even gets off a shot, dropping him to the ground ... and, other audience members have to pull the guy off.

Turns out the guy in the green pants is actually Antonio Barrul -- a six-time amateur Spanish boxing champion who went pro last year -- the worst type of random person to throw hands with, we gotta say. He was interviewed in the local news there in Leon, and explained what happened (from his POV, anyway).

In the clip, Barrul apologizes to the theatergoers for the fight ... and he's since said publicly he regrets his actions -- wishing he'd waited for security to come and handle the guy instead of taking matters into his own fists.

Antonio says he had a good reason to get involved though ... 'cause he claims the other guy was screaming at a woman he was with at the film, and he got up to handle the situation.