An Independence Day celebration at a packed Utah football stadium descended into chaos when a firework was shot into the crowd, sending up to six people to the hospital.

The incident occurred during the opening ceremony of the Stadium of Fire event at the Freedom Festival in Provo, which was held at LaVell Edwards Stadium on the campus of Brigham Young University.

According to Emory Cook, a spokesman for the Freedom Festival, the show was halted for about 15 minutes while injured audience members were treated. While six people were taken to the hospital, details on the total number of people hit by the fireworks and the extent of their injuries were not immediately clear.

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Authorities are currently investigating the cause of the incident, which occurred when fireworks were set off inside the stadium while jet planes flew overhead. Social media videos showed some of the fireworks deviating from their intended trajectory and landing in the stands of the open-air arena, where around 45,000 people had gathered for the sold-out spectacle.

“Clearly there was a fireworks misfire, but we are still trying to figure out how that happened,” Cook said.

Teresa Jack of Provo, who witnessed the terrifying scene from the field, described how a missile flew over her head before exploding in the east stands, creating a bomb-like cloud of smoke.

According to one spectator, chaos broke out when people in the crowd signaled for help and shouted “Stop the show” into the stage loudspeaker.

Ms Jack, who was present at the event, praised the well-coordinated measures that helped prevent a stampede, pointing out that people were instructed to remain seated while the injured were directed to seek medical attention in a designated tent.

Jeanie Atherton of the Provo Fire and Rescue reported that her team, which was on standby at the beginning of the incident, was able to help the injured within a minute.

Although only one person from her department was taken to hospital, Atherton acknowledged that others may have sought medical attention on their own.

The concert, headlined by the Jonas Brothers, went ahead after a safety clearance from the Provo Fire Marshal and a thorough inspection of the fireworks, BYU police spokeswoman Karen Ellingsworth confirmed.

Cook said the fireworks that failed to work inside the stadium were small in scale compared to the pyrotechnics for the grand final, which are stored outside the venue.