HOUSTON, United States (AP) — At least eight people died and numerous others injured in what officials described as a surge of the crowd at the Astroworld music festival in Houston, Texas while rapper Travis Scott was performing on Friday night.

Officials declared a “mass casualty incident” just after 9 pm during the festival where an estimated 50,000 people were in attendance, Houston Fire Chief Samuel Peña told reporters at a news conference.

“The crowd began to compress towards the front of the stage, and that caused some panic, and it started causing some injuries,” the fire chief said. “People began to fall out, become unconscious, and it created additional panic.”

Officials transported 17 people to hospitals, including 11 who were in cardiac arrest, Peña said. It wasn’t clear whether all eight who died were among the 17 that had been transported to hospitals. Many people were also treated at the scene at NRG Park, where a field hospital had been set up. About 300 people were examined at that site throughout the day, he said.

Event promoters had arranged for medical units to be on scene at the festival, however once the crowd surge began, those units were “quickly overwhelmed,” Peña said.

In a video posted to social media, Scott could be seen stopping the concert at one point and asking for aid for someone in the audience: “Security, somebody help real quick.”

Peña said officials did not immediately know the causes of death for the eight people who died. A medical examiner would investigate. The deceased had not been identified as of early Saturday.

The show was called off shortly thereafter.

Astroworld is a two-day music festival that was scheduled to take place Friday and Saturday in Houston. The event was sold out, according to the Astroworld website. Saturday’s performances have been cancelled.