UFC Fight Night 237 saw some big names throw down in Mexico City … but the biggest knockout of the night may have actually happened outside the Octagon.

Video of a brawl breaking out in the crowd during the big event Saturday night is going viral online … with a huge group of men grabbing each other by the shirt and throwing punches, essentially turning the crowd into a mosh pit.

Right in the middle of the melee … one of the biggest knockouts in UFC history — if you can count a fight going down in the stands that is.

Check out the vid … a man in a black shirt throws a vicious left hook against his unsuspecting victim, and the guy’s body basically goes limp as he hits the concrete canvas.

The fight sorta cools off after the KO … with guys on both sides holding people back — though they don’t look like security guards per se.

In fact, it seems like security took forever to respond — just check out the multiple angles of the fight that go on for a while … and UFC prez Dana White even expressed his shock at the lack of a security presence.

White said he stood there watching the fight for a while, just waiting for security to come in and handle the fight … but they seemed to just let it go until it ended.

DW’s clear though — he doesn’t feel it reflects poorly on the UFC … even if it was one of the craziest things he’s ever seen, he said.

BTW … several fights actually went down in the ring too with Brandon Royvaldefeating Brandon Moreno to launch himself back into UFC Flyweight title contention.

A pretty successful night in UFC … even if some fans got a closer look at the action than they were expecting.