At least seven people were taken to hospitals with non-life-threatening injuries, the West Metro Fire Department said in a tweet. Up to 90 people were treated on scene, the department said. Injuries included cuts and broken bones, it said.
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Bella Oliveira said she was almost trampled as she and others ran from the falling hail.
"I managed to hide where they were selling drinks and I used my backpack to protect myself and another girl," she said, adding that she hurt her head and shoulder during the incident.
The National Weather Service office in Boulder first issued a severe thunderstorm warning around 9:04 p.m. Wednesday that included the amphitheater. The weather service warned the storm was capable of producing winds up to 50 mph and golf-ball-sized hail.
As the storm approached, the weather service office tweeted this alert, even tagging the amphitheater that dangerous weather was imminent.
Earlier in the day, the Boulder weather service office warned that "We are still in the 'loaded gun' scenario of having tremendous instability," which could lead to severe weather throughout the evening.
The Storm Prediction Center warned of a level 2 out of 5 "slight risk" of severe weather for Red Rocks and a level 3 of 5 "enhanced risk" for the nearby Denver area.
The forecast said the area could experience damaging winds, large hail and tornadoes throughout the afternoon extending into the overnight hours. Six tornadoes were also reported across Colorado on Wednesday evening.
Former One Direction star scheduled to play
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Musician Louis Tomlinson was scheduled to play at the Red Rocks concert.
"Devastated about the show tonight, hope everyone's ok, I'll be back!" the former One Direction star tweeted. "Even though we didn't play the show I felt all of your passion! Sending you all love!"
The performance was postponed twice due to weather conditions, with the venue telling fans to take shelter in their cars before giving an "all clear" that brought fans back to the venue, only to finally postpone the show and tell fans to go home.
CNN has reached out to the city and county of Denver, which owns and operates Red Rocks Amphitheatre, for comment.
One of the concertgoers, Ashlie Scott, described a chaotic scene.
"The minute they announced it was an extreme weather warning and we needed to seek shelter now, the sky opened up and it started hailing and pouring rain," Scott told CNN.
"It was almost like a crazy movie scene," Scott said. "The sky was lighting up with lightning, so I was expecting a quicker extreme weather warning besides a delay."
Video taken by Scott shows hail coming down a staircase as lightning flashes in the sky and alarmed concertgoers pass by. Four large hailstones are seen in Scott's hands.
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Another video from the scene shows a group hiding in a crowded bathroom during the hailstorm.
"About 10 minutes before the hail started really coming down, I got hit with a small piece of it, and didn't realize it was hail until it started coming down on us out of nowhere," concertgoer Jessie Mueller, who took the video, told CNN. She and her friends found shelter in the bathroom, she said.
"We are unharmed, but some of my other friends got caught in it and now have broken bones and bruises," Mueller said. "I saw just piles and piles of hail, and it looks like it had snowed in the seats of the amphitheater when we finally got out of the bathroom."
At least 11 tornado reports were made Wednesday across the western and central US, including the six in Colorado, three in Texas, one in Wyoming and one in Nebraska, according to the prediction center.
Four people were killed in a northwestern Texas town when a tornado struck it Wednesday, officials said, as storms pummeled parts of the western and central US.
The tornado struck Matador, a town of a few hundred people roughly a 280-mile drive northwest of Dallas, Wednesday evening, the National Weather Service office in Lubbock said.
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