Two workers helping to set up the main stage for the Ultra Music Festival, which begins Friday afternoon in downtown Miami, were critically injured Thursday night when a giant LED screen fell, trapping the men underneath it.

Two of the men suffered what fire rescue officials said were life-threatening injuries, including one with two possibly broken legs.

Two other workers were also struck by the falling equipment; one was in stable condition, and the other was treated at the scene. The seriously injured workers were taken to Jackson Memorial Hospital.

The 8 p.m. accident, which occurred on the eve of what is billed as “the largest electronic dance event in the world,” will force a section of the large concert area at Bayfront Park to be closed off to the thousands of concert-goers who are expected.

But the popular music event, which this year extends over two weekends and has six other stages, will kick off as scheduled at 3 p.m. Friday.

“Festival organizers are working with and supporting authorities as they investigate the details behind the accident,” the festival said in a late-night news release Thursday. Tickets for the weekend start at $299.

Miami Fire Rescue spokesman Ignatius Carroll said the fire marshal and investigators would return to the scene in the daylight to begin working to determine what caused the accident. OSHA will also be on the scene.

The accident apparently occurred as giant screen, or jumbotron, was being hoisted up in the air to be part of the stage. Instead it crashed down on the workers.

Among those scheduled to perform at this year’s annual fest, its 15th, are David Guetta, Swedish House Mafia, Deadmau5, Avicii and Snoop Dogg.

Miami Herald staffers Noel Gonzalez and Greg Castillo contributed to this report.