Fans watching the game on TV were not aware of what was going on in the stands of Buck Shaw Stadium at Santa Clara University.
At least 20 Santa Clara police units were called to the stadium, and seven fans were arrested. Authorities said five are being charged with resisting arrest.
"I heard yelling and screaming, so I looked back and I just saw pushing and shoving, just a big wild group out of control," said witness Dylan Hill.
In the middle of the commotion, at least one smoke bomb went flying. Witnesses said it came from the Galaxy section, but that has not been confirmed by police.