Crowds surging forward as rapper Travis Scott took to the stage at the Astroworld Festival in Houston led to chaos and crushed bodies, people who escaped the tragedy on Friday night said.
At least eight people died and scores were injured during the first day of a weekend event that attracted tens of thousands of revellers.
Police have opened a criminal investigation. About 300 people were treated for injuries, including 11 taken to hospitals suffering from cardiac arrest.
Witnesses said fans were crushed and trampled at times during the day, before the main act took the stage.
The crowd was already on edge and throwing water bottles when faced with choke points at a Covid test centre and merchandise sales areas, festivalgoers told news agency Reuters.
There was "chaos" throughout the day as crowds overwhelmed the facilities and security on site, said Anthony Mendiola, 22, from San Antonio, Texas. While police and security teams were present throughout the park, "there just wasn't enough," he said.
There were 528 Houston police and 755 private security members hired by festival organiser Live Nation working at the event, Houston Mayor Sylvester Turner said at a briefing on Saturday.
“It felt like a riot. There were people throwing out stuff, objects flying everywhere,” said Albert Merza, 43, a supermarket manager, who saw fans attempting to jump fences to get inside and a "lot of crazy behaviour."
Countdown clock to Travis Scott performance causes crowd surge
A countdown clock that signalled headliner Travis Scott was to perform that night pulled scores towards the main stage, crushing fans at the front, said Jonathan De Leon, who moved away as fans up front were pushed from behind, some falling to the ground.
Job Ramos, 21, from Kennewick, Washington, had also attended last month's Rolling Loud festival in New York, which billed itself as the world's largest rap festival. Houston's had more people, he said, and the mosh pits where fans dance and jump were frightening enough to keep the regular concertgoer away.
Pablo Longoria, 19, a college student from Houston, said the crowd erupted when rap superstar Drake appeared unexpectedly on stage with Travis Scott.
"All of a sudden it was like there was 13 mosh pits," said Mr Longoria.
He recalled people falling into piles, including one woman who fell to the ground and was covered by three others who toppled after her.
"It's not easy getting out of those mosh pits," said Mr Ramos.
"When you're in a mosh pit, you feel like you can't get oxygen. All you are breathing is CO2."