California fire officials say at least 21 people were treated for burn injuries after trying to walk on hot coals at an event hosted by motivational speaker Tony Robbins.

Robbins, an American self-help author, was hosting a four day clinic called "Unleash the Power Within" on Thursday at the San Jose Convention Center. Witnesses say attendees gathered in a nearby park where 12 lanes of hot coal, measuring 10 feet long and 2.5 feet wide, were laid out on the grass, according to San Jose Fire Department Capt. Reggie Williams.

Robbins tried to walk on the coals and encouraged his participants to do the same.

That's when at least three people were sent to the hospital, and most suffered from second or third-degree burns.

One witness told the San Jose Mercury News he heard people screaming when he walked up to the scene.

"It was people seriously hurting, like they were being tortured," 25-year-old Jonathan Correll said. "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."

Williams said event organizers had a city burn permit authorizing the coals. A fire prevention inspector was also on site to make sure no fire occurred.

More than 6,000 people were in attendance at the event.

Robbins Research International released a statement Friday, saying, "We have been safely providing this experience for more than three decades, and always under the supervision of medical personnel. We continue to work with local fire and emergency personnel to ensure this event is always done in the safest way possible."