The disaster unfolded at NRG Park at around 9pm on Friday during the headline performance by Scott, a Grammy Award-nominated singer and producer.
As some in the sold-out audience of about 50,000 surged toward the stage, people began to fall unconscious, some apparently suffering cardiac arrest or other medical issues, officials told reporters outside the venue.
Minutes later the chaos was declared a "mass casualty incident."
"It happened all at once. It seemed like it just happened ... over the course of just a few minutes," said Houston Police Executive Assistant Chief Larry Satterwhite.
After mobbing entrance gates and merchandise booths early in the day, the crowd grew increasingly unruly as performers took the stage, according to 19-year-old festival goer Hamad Al Barrak.
"There were just too many people,' said Al Barrak, who described chaos as he tried to buy festival gear. "People were up on each other. We were all pressed together. You felt like you couldn't breathe."
Houston-born Scott said he would support the police as they investigate the incident and wanted to help the community heal.
"I'm absolutely devastated by what took place last night. My prayers go out to the families and all those impacted by what happened at Astroworld Festival," Scott wrote on Twitter. "Love You All."
Scott stopped multiple times during his 75-minute performance when he spotted fans in distress near the stage, video posted online showed.
He asked security to make sure they were safe. Emergency vehicles, lights and alarms flashing, cut through the audience several times.
"We need somebody to help. Somebody passed out right here," Scott said, according to video of his set, which included a guest appearance by Canadian rapper Drake. "Security, somebody, jump in here real quick."
In an interview with CNN, Houston Fire Chief Samuel Peña described how the crush of fans quickly overwhelmed the private security hired for the festival, a two-day affair that was scheduled to culminate on Saturday. The second day of the event was cancelled.
Authorities transported about 23 people in serious condition to hospitals, 11 of them critical, Pena told CNN. One of the injured was 10 years old. At least eight people succumbed to their injuries, he said, adding that the death toll could rise.
It was not yet clear what caused the disaster. An investigation was underway that is likely to focus on security and safety protocols at the event.
Two weeks ago, organisers of another Houston concert canceled it abruptly after fans pushed through a gate and stormed into a Playboi Carti show being held within the same NRG Park complex.
