According to the San Bernardino County Sheriff's Department, thirty people were arrested during the first day of Ozzfest Meets Knotfest at San Manuel Amphitheater in Devore, California. Ten other attendees were transported to the hospital for various medical reasons.
Most of the arrests on Saturday, September 24, were alcohol-related.
Uniformed and undercover law enforcement officers from the California Highway Patrol and San Bernardino County Sheriff's Department were deployed, along with K-9's.
About 37,000 people attended the event on Saturday, with 3,400 campers on the festival grounds.
Saturday's Ozzfest lineup was headlined by BLACK SABBATH, while SLIPKNOT closed out Sunday's Knotfest bill.
SABBATH singer Ozzy Osbourne and bassist Geezer Butler were asked what they thought such a massive show said about the state of rock music. "I don't know about the state of rock music, but I'm just gonna go up there and have some fun, you know," Ozzy said. "We're not trying to make a statement to the industry, we're just gonna have some fun." Geezer added: "It's good that we're still going and this kind of music's still going. To do something this big proves that it's still as popular as ever."
Also on the Ozzfest portion of the weekend were DISTURBED, OPETH, BLACK LABEL SOCIETY and others, while the Knotfest lineup included SLAYER, TRIVIUM, ANTHRAX, SUICIDE SILENCE and many more.
Both days featured three stages, with one of them named after late MOTÖRHEAD frontman Lemmy Kilmister.
The San Manuel Amphitheater and Festival Grounds is where Ozzfest began in 1996 and where Knotfest was staged last year.
Saturday's show was the last ever for BLACK SABBATH in California. The band is in the final months of its farewell tour and will play three more U.S. shows in Texas this November before visiting South America. The legendary act will end its run with a series of U.K. gigs in early 2017.
Ozzfest started 20 years ago and was the first national music festival of its kind dedicated to hard rock music.
Ozzfest has not been a traveling festival in the U.S. since 2007's "free" edition. The show morphed into a one-off event in Dallas in 2008, took 2009 off and played just six cities in 2010.
Knotfest, meanwhile, staged its third annual event last year and second as a Southern California weekend destination festival. It attracted over 50,000 fans with 50 bands playing on four stages.