Twenty-five people have been treated for burns after walking on hot coal in northern Switzerland, according to police.

Emergency services were alerted to injuries at a private event on the Au peninsula on Lake Zurich Tuesday evening, Zurich canton (state) police said in a statement.

They said that 25 people were given medical treatment at the scene, and 13 of them were hospitalized with more severe injuries. Investigators secured evidence and started an investigation into the circumstances of the incident.

There was no immediate word on why people at the event walked over hot coal.