Fire officials in California say at least 21 people were treated for burns after attendees of an event for motivational speaker Tony Robbins tried to walk on hot coals.

The San Jose Mercury News reports that at least three people went to a hospital and most suffered second- or third-degree burns.

Jonathan Correll, 25, told the paper that he heard "screams of agony."

"It was people seriously hurting, like they were being tortured," he told the paper. "First one person, then a couple minutes later another one, and there was just a line of people walking on that fire. It was just bizarre, man."

Robbins was hosting a 4-day gathering called "Unleash the Power Within" at the San Jose Convention Center. Witnesses say on Thursday, a crowd went to a park where 12 lanes of hot coals were on the grass.

Robbins' website promotes "The Firewalk Experience" in which people walk on super-heated coals.

Fire Capt. Reggie Williams says organizers had an open fire permit and emergency personnel were on standby.

Other attendees reportedly successfully walked over the coals and called the experience life-changing.

"Overcoming something like that, it's a breakthrough," Henry Guasch, 19, told the paper. Guasch said he suffered a minor burn.

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The Associated Press contributed to this report