Several Giants and 49ers fans got into such a wild fistfight in the stands following their game Thursday night ... at least two people were left bloodied.
The brawl went down just minutes after San Francisco beat New York at Levi's Stadium in Santa Clara, Calif. ... after a witness said fans had been trash-talking so much, one of them threw a beer that landed on a woman.
According to the bystander, as soon as the liquid fell on the lady ... a melee immediately broke out -- and in footage from the scene, obtained by TMZ Sports, you can see things got violent in a hurry.
One fan wearing a Joe Montana jersey was punched in the face so many times, his nose gushed blood.
Just feet away, two Giants fans were seen throwing haymakers at a guy in Niners gear ... hitting him so flush in the face, a cut opened above his right eye.
To the SF supporter's credit, he fought off the men ... landing several punches of his own -- before bolting from the area.
One witness tells us authorities did ultimately come to the stands to address the fracas ... though by the time they arrived, nearly all of the combatants had left.
A spokesperson for the Santa Clara Police Dept., however, tells us they are investigating the incident further.
The fight was hardly the only one at the stadium on Thursday ... just a few hours before -- a man was seen pulling a woman's hair during a scuffle in a different part of the venue.