A well-known Mexican wrestler who co-starred in the comedy “Nacho Libre” has died of a suspected heart attack after collapsing on stage in London.

Cesar Cuauhtemoc Gonzalez Barron, the lucha libre star known as Silver King, performed Saturday night at The Roundhouse. The north London venue said Sunday on Twitter that he died during the event and more information about the circumstances are being pursued.

The 51-year-old Gonzalez appeared as a champion wrestler and comic villain in the 2006 movie “Nacho Libre”, which starred Jack Black as a monk who wants to be a masked luchador. Lucha libre is a popular form of wrestling in Mexico that features colorful masks, elaborate costumes and acrobatic techniques.

Lucha Libre World, which promoted the Roundhouse event, said in a statement the star “suffered what we believe was a cardiac arrest while performing in the show and sadly passed away.”

Attendee Roberto Carrera Maldonado told the BBC the wrestler’s condition initially appeared “staged” as part of the fight, before efforts were made to revive him.

“All of us were really shocked. It wasn’t clear what was happening,” Maldonado said. “I had the impression they didn’t know what to do.”

The London Ambulance Service said medics arrived five minutes after they were called to the Roundhouse on Saturday, but the patient was pronounced dead at the scene.

Silver King competed in World Championship Wrestling between 1997 and 2000.