At least two investigations, one of them criminal, were underway today into the deadly stampede during rap star Travis Scott's Astroworld music festival that killed at least eight people and injured dozens.

Officials in Houston, Texas, where the event took place, said autopsies on Friday's victims were being performed as soon as possible so their bodies could be returned to family members.

Among the youngest victims were two children aged 14 and 16. Two of the dead were aged 21, another two were 23, with a seventh aged 27.

The eighth victim has yet to be identified, Houston Mayor Sylvester Turner told reporters.

Mr Turner said authorities are looking at video footage, talking to witnesses, concert organisers and people who were hospitalised.

"This is a very, very active investigation, and we will probably be at it for quite some time to determine what exactly happened," he said.

Harris County Judge Lina Hildago called for an "objective, independent" investigation into the tragedy, as she spoke about the rap festival being attended by 50,000 fans when the stampede took place.

"Perhaps the plans were inadequate. Perhaps the plans were good but weren't followed," Ms Hildago said.

"The families of those who died, everybody affected, deserves answers."

Houston city police chief Troy Finner said his department had opened a criminal investigation by homicide and narcotics detectives, following reports that somebody in the audience had been injecting people with drugs.

Houston Police Chief Troy Finner speaks to the media

The city's fire chief, Samuel Pena, said several concertgoers had to be revived with the anti-drug overdose medicine Narcan, including a security officer who appeared to have been injected in the neck with a substance by a concertgoer.

The rest of the two-day festival has been cancelled after the disaster unfolded at NRG Park at about 9.30pm during the headline performance by Travis Scott, a Grammy nominated singer and producer, at the climax of the opening day.

The deaths happened near the stage when the crowd surged towards it, with some suffering cardiac arrests and other medical trauma, officials said.

Survivors described chaotic scenes of people squeezed up against one another with many struggling to breathe.

Gavyn Flores, 18, said he was standing on the edge of the crowd near a barricade and could not move, for hours on end. He said he tried to hoist people over that wall.

"People were trying to get out, but you can't move. So there kind of wasn't a point of trying to get out, because they couldn't. But if they could, we were trying to help them get thrown over," Mr Flores told AFP.

The two-day festival has been cancelled

"I am absolutely devastated by what took place last night," Travis Scott tweeted Saturday. "My prayers go out to the families and all those impacted by what happened at Astroworld Festival."

More than 520 police officers and 750 security guards were on hand for the festival.