Fire officials say 21 people at an event hosted by motivational speaker Tony Robbins suffered burns while walking across hot coals, with three of the injured treated in hospitals.
The injuries took place during the first day of a four-day event at the San Jose Convention Centre hosted by Robbins called Unleash the Power Within. Most of those hurt had second and third degree burns, San Jose Fire Department Captain Reggie Williams said.
Walking across hot coals on lanes measuring three metres long and heated to between 1,200 to 2,000 degrees provides attendees an opportunity to "understand that there is absolutely nothing you can't overcome", according to the motivational speaker's website.
Robbins Research International said in a written statement that 6000 attendees of the event walked across the coals on Thursday.
Organisers had an "open burn permit" and medical staff at the event, and there was also a fire inspector on the scene, Williams said.
"Once they (the medical staff) became overwhelmed, our inspector called for us," Williams said.
Witness Jonathan Correll said: "I just heard these screams of agony. People were in pain. It sounded like people were being tortured."
Correll, 25, of San Jose, said he saw three ambulances, about 10 to 15 people on the ground being treated by paramedics and some people being wheeled away on stretchers.
"It was really just chaos," he said.
Participant Sahar Madani told KTVU-TV that attendees were warned that they might get burns or blisters.
"The intention of the event is to get your focus and your attention away from that and look into the power within yourself and focus on just walking on the fire," she told the station.
Robbins, 52, who bills himself as an entrepreneur and "peak performance strategist," has written five books that are published in 14 languages, and has sold more than 40 million audiotapes of a professional development system dubbed Personal Power, according to his website.