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A pack of vicious and cowardly 49ers fans bashed in a Vikings fan's face during a chaotic melee outside the Monday night game, and if not for one brave security guard ... the Vikes fan might be dead.

The video of the brawl is heinous ... the Vikings fan is already down on the ground outside Levi Stadium when the Niners backers continue unloading on him -- kicking and punching him in the head and body. It's brutal.

But you also see the guard run into the middle of the fray -- even though he's way undersized -- and scramble around the victim. He eventually succeeded in breaking up the fight.

Honchos at the security company tell TMZ Sports their staffer, who was Johnny-on-the-spot, is one of their star employees ... so they're not shocked he stepped up. They also say they're planning to honor his heroism.