Fans of the Canadian DJ duo Zeds Dead were disappointed Saturday night when the concert was shut down because several in the audience had to be hospitalized because of drug use.

On the band's Facebook page, several fans said they had driven from Massachusetts, Baltimore, and elsewhere to attend the rave. Police were called at 8 p.m., before the band even went onstage.

When officers arrived at the Electric Factory at Seventh and Callowhill Streets, they found at least four people who had overdosed on an unidentified drug, according to the Associated Press.

Initial reports said as many as nine people, mostly teens, some as young as 15, had passed out. Those reports could not be verified Sunday.

It wasn't clear what drugs were being used, whether they were tainted or too strong or both. But many concertgoers on Facebook cited so-called bath salts and Ecstasy, and lamented "this is why concerts should be 18+."

Cat Lepiane, 18, said that later in the evening, the crowd was rowdier than usual for dubstep, a genre of electronic dance music. Dubstep concerts, she said, are more about dancing than moshing. But on Saturday, there was "a lot of crowd-surfing, and I saw a bunch of people being pulled out of the crowd," she said. "Everybody was pushing each other, and it's not usually like that."

She and a friend pushed their way to the front door when they heard the show was being canceled. Outside, she saw six ambulances and several police cars.

"There were a bunch of people standing near the ambulances, crying, like their friends were the ones who overdosed," said Lepiane, a senior at North Penn High School. She estimated there were about 20 people standing around, and they appeared to be about 17 years old.

Lepiane said she didn't see anyone selling drugs or overdosing.

"I saw a couple of people around me doing pills and stuff, but I didn't ask. One kid had a bloody nose and he said it was from snorting, so that was kind of crazy," she said.

The Electric Factory is open to all ages, but access to the two bar areas is restricted.

After a couple of hours of confusion, Zeds Dead - the band's name is a reference to a Bruce Willis line in the film Pulp Fiction - announced on Twitter that they t had moved the show to the Lit Ultra Lounge on North Second Street, where patrons must be at least 21.

A statement released Saturday night by Deathwaltz Media Group, one of the marketing companies the Electric Factory hired to promote the event, described the news of hospitalizations as "very sad," and said it hoped "that everyone can learn from this and start enjoying these types of concerts without the use of drugs."

Justin Berger, a Deathwaltz spokesman, said that the company wasn't affiliated with the Electric Factory and that he had heard about the overdoses through social media.

The Electric Factory did not respond to requests for information. Police had no information about a possible investigation.