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.