49ers Fans Get In Massive Brawl at Preseason Game, One Knocked Out -

FRIENDLY FIRE

It's not often you see fans cheering on the same team get in a nasty brawl -- but that's exactly what happened in San Francisco -- with haymakers flying ... and even a knockout!!!

Unclear what led to the massive melee Saturday night at Levis Stadium as the 49ers hosted the Denver Broncos in a preseason matchup, but it got messy fast. You can see lots of pushing, shouting and confusion before blows are exchanged.

At one point, a guy in a Jerry Rice jersey starts absolutely unleashing some fists on a dude in a white t-shirt, connecting several times with his face.

One of the blows appears to knock the dude out, sending him into a chair. After things calm down, the same guy -- wobbly on his feet -- gets up with blood on his mouth and shirt.

Event staff leads the battered dude up and out of the section, while he struggles to take on the stairs.

TMZ INVESTIGATES

We don't yet know if there were any arrests in connection to the brawl, but it sure made for some excitement in the stands. As for the games ... the 49ers won 21-20.