AT least 21 people have been treated for burns after attendees of an event by motivational speaker Tony Robbins tried to walk on hot coals, fire officials in California say.

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

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

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

Witness Jonathan Correll says he heard "screams of agony".

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

Originally published as Feet burned at Robbins motivational event