That's when police got several calls at around 10:45 p.m. of cell phone thefts and fights breaking during the park's "Halloween Haunt", a popular nighttime event held in the weeks leading up to Halloween.
Officers made two arrests: one for theft by a juvenile and one for public intoxication.
Santa Clara Police say it was difficult to identify the perpetrators because most scattered by the time officers arrived.
They plan to review security video in hopes of identifying and arresting more people. Police say a group of about a hundred juveniles were responsible for Saturday night's problems.
Jill Neely of Oakland picked up her daughter and friends from Great America Sunday afternoon. She said she's not surprised to hear about what happened last night.
"Actually my daughter wanted to go," said Neely. "I would never let my daughter go to Great America in the nighttime because I heard this kind of stuff happens too much and I'm just too worried about that so I made them go to the Pumpkin Fest today."
Great America provided ABC7 with the following statement:
"There were incidents on October 28th at California's Great America that required assistance from on-property Santa Clara Police Officers and were quickly addressed. The safety of our guests is our top priority."