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

The San Jose Mercury News reported that at least three people were taken to hospital, with most suffering second- or third-degree burns.

Robbins was hosting a four-day gathering called "Unleash the Power Within" at the San Jose Convention Centre. Witnesses said that a crowd went to a park where 12 lanes of hot coals were set up on the grass.

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

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

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

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