Injured spectators were stretchered away from the stage at Astroworld Festival
Panic broke out after the crowd began to press towards the front of the stage at rapper Travis Scott's Astroworld Festival, emergency officials said.
Eleven people were taken to hospitals in cardiac arrest and eight died. Some casualties are aged as young as 10.
About 50,000 people attended the event, which has now been cancelled.
Lina Hidalgo, the county judge of Harris County, described what had happened as an "extremely tragic night".
"Our hearts are broken," she said. "People go to these events looking for a good time, a chance to unwind, to make memories - it's not the kind of event you go to where you expect to find out about fatalities."
IMAGE SOURCE,GETTY IMAGES
Rapper Travis Scott stopped performing when he saw fans in distress
The incident began around 21:15 on Friday (02:15 GMT Saturday), Houston Fire Chief Samuel Peña said.
"The crowd began to compress toward the front of the stage, and people began to panic," he told reporters.
As the crush began causing injuries to people, the panic grew, the official added.
According to the Houston Chronicle, Travis Scott, who founded the festival, stopped multiple times during his 75-minute performance when he spotted fans in distress near the front of the stage.
The rapper asked security to make sure they were okay and help them out of the crowd. Emergency vehicles, lights and alarms flashing, cut through the crowds several times, the paper says.
Houston Fire Chief Samuel Peña said there was panic when people "began to compress towards the front of the stage."
Amateur video from the concert appears to show fans jumping on an emergency vehicle in the crowd as the performance continues on stage.
Organisers halted the event when it was apparent that many people had been hurt, but the sheer scale of the casualties quickly overwhelmed the existing medical facilities, the fire chief added.
Fans pressed up against each other near the front of the main stage, eyewitnesses told the Chronicle. In some places, people were so crammed together that they started hyperventilating or struggling to leave.
"We were hanging on to each other to avoid getting separated," said a male concert-goer. "If you let go, you could easily drift apart."
A female concert-goer told the paper she had never been to a "music festival with so many angry people".
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US rapper Lil Baby also performed at the festival
Some 300 people were treated for injuries such as cuts and bruises on Friday. Judge Hidalgo said "many" of those killed at the festival had been "tragically young".
A reunification centre has been set up at the Wyndham Houston Hotel for families who have not heard from loved ones at the festival.
Earlier on Friday, hundreds of people had rushed the event's perimeter, knocking down metal detectors and a security screening area to get into the concert, news outlet ABC13 reports.
Saturday was to have been the second and final day of the festival.
"Nobody could dream of this," said Houston Police Department Chief Troy Finner. "But we're here, and I think it's very important that none of us speculate. Nobody has all the answers tonight."
By Joey Guerra, music critic for the Houston Chronicle, spoke to BBC Radio 5 Live
I had been there for most of the day and there had been minor incidents throughout most of the day. When the gates opened people stampeded through and jumped metal detectors, and a few people got injured that way. I saw a couple of times people being taken out on stretchers. I'm not sure what the reasons were.
During Travis Scott's set there were multiple times when you could see emergency vehicles with their lights on going through the crowd, but they would cut through and the show would go on. Travis Scott himself did stop the show several times to point out people near the front who were in distress or needed help and he would get security to come and help them. I don't think he was aware of the extent of what was going on.
Travis Scott is a rapper from Houston so this was particularly special for our city.
This is the third year of the festival. It is kind of an homage to an amusement park called Astroworld that was very popular here in Houston and that is not around anymore. He has kind of re-imagined this whole concept. There are carnival rides, games, different things that people who knew Astroworld back then are going to recognise here, and it has gotten bigger every year.
Live Nation, the event organiser, did not immediately respond to a request for comment, Reuters news agency reports.
Astroworld is an annual event held at NRG Park since 2018, though it was put on pause last year because of the Covid-19 pandemic.
Travis Scott, 29, made his big breakthrough in 2013 and has since had eight nominations for Grammy Awards. He has a child with celebrity socialite Kylie Jenner.
Other acts scheduled at the festival over the weekend included rappers Chief Keef and 21 Savage, and Australian rock act Tame Impala.