A 28-year-old female circus performer has fallen 11m and seriously injured herself during a Circus Rio show in the Adelaide suburb of Glenelg.

Horrified kids on school holidays initially thought the trapeze accident at 4.45pm was part of the show, but paramedics soon rushed to the scene.

Witnesses say children could be heard crying after they realised it wasn't part of the show.

An SA Ambulance spokeswoman told news.com.au the performer was urgently transferred Royal Adelaide Hospital with "multiple and very serious injuries" — although she couldn't yet confirm what type of injuries the patient sustained.

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Spectator Alice Young told ABC the incident happened about five or six acts into the show, while the woman was performing on an aerial hoop.

"She had the ring behind her neck and the next thing, she just went bang to the ground, straight on wood," she said.

"We all thought it was part of the act and after about 10 seconds we realised it wasn't.

"People came running and then we were asked to leave."

The acclaimed Rio Circus at Wigley Reserve draws its inspiration from Brazil's famous Carnival.

It features samba dancing, aeroskills and a 23m freestyle motocross bike jump.

It's advertised as a global affair, with performers from Brazil, France, Russia and beyond.

Just an hour ago, event organisers shared a Facebook post promoting a return of the trapeze show.

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"Our Trapeze show is BACK!" it read.

"Catch our team of Russian and Belarusian trapeze artists on NOW at Wigley Reserve, Glenelg!"