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 Royval defeating Brandon Moreno to launch himself back into UFC Flyweight title contention.