Bands of violent teens turned the Halloween Haunt at California’s Great America into a real-deal fright fest over the weekend, leaving many visitors to the Santa Clara amusement park soured on the holiday event and even a little banged up.

Park officials acknowledged “there were incidents” on Saturday that “required assistance” from police, but they did not specify what exactly went down as thousands visited the Halloween-themed attractions.

Police and witnesses detailed upwards of 100 teens and young adults harassing, robbing and assaulting guests in the late-night hours Saturday. The scene unfolded around 10:45 p.m. Saturday when Santa Clara police got reports of the unruly youths causing disorder. Officers later arrested one juvenile on suspicion of theft.

Many park-goers fled in the chaos, describing their frightful experiences on social media.

“Got punched in the face, evacuated out from mazes and it was just all bad,” @dancerallim wrote on Twitter.

Twitter user Ana Banana shared a similarly chilling account: “First we thought it was part of the show but then they told us to run for our lives.”

Most of the attackers bolted or blended into the crowds before police arrived, said Lt. Dan Moreno, a Santa Clara police spokesman.

“They fled in different directions. It was dark so it was hard to track down people,” he said. “We’re reviewing surveillance.”

Roughly 20,000 people packed into the park Saturday, visiting attractions like the “Backwoods” haunted swamp, the “Chaos House” and the “DEMON: RE-IGNITED!” roller coaster. The Halloween event was billed as “the fright of your life.”

The park said it “quickly addressed” Saturday’s incidents, and the final day of Halloween festivities on Sunday went off without problems.

“The safety of our guests is our top priority,” California’s Great America said in a statement.

Staff writer Kurtis Alexander contributed to this report.

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: Twitter: @EvanSernoffsky