Here's one of the craziest and most violent in-arena brawls EVER -- courtesy of some fans at Thursday night's L.A. Kings game.
The hometown squad was in a tight matchup with the Florida Panthers ... when one witness tells us he noticed a fight beginning to break out in the Staples Center lower bowl.
You can see in the video what happens next ... a scrap escalates into AN ALL-OUT BRAWL!!
The scene is insane ... there's a woman throwing a FLURRY of right hands to start -- and then things get even crazier.
A shirtless man runs into the scrum with a flying left haymaker .