The reason for the brawl: a parent didn't like the calls being made by a 13-year-old umpire.
The fight began at Westgate Elementary School in Lakewood, Colorado, a suburb southwest of Denver, on Saturday at about noon as 15 to 20 adults got into a violent tussle, according to Lakewood police.
The brawl was still ongoing as Lakewood police arrived at the scene.
Police issued four citations for disorderly conduct, but said they are still searching for others involved in the fight.
Police are looking for an adult in a white T-shirt and teal shorts in particular because he can be seen in the video throwing sucker punches at people looking in the other direction. Police said it is unknown if this person is a parent of one of the children in the game.
There were a few minor injuries and one person suffered serious bodily injury, police told ABC News. No details on the injury were available.
ABC News' Timmy Truong contributed to this report.