A woman was killed and at least seven others including a one-year-old child were injured when a car competing at a demolition derby in Montana drove over a chain-link fence into spectators, authorities said.

A woman in her 30s died at the hospital in Deer Lodge, according to Powell County Sheriff Gavin Roselles, who witnessed Sunday’s crash and aided the injured. Her name was not released.

Demolition derbies have been a staple at American racetracks and county fairs since the Second World War, and remain popular to this day. They usually involve any number of generally old cars repeatedly bashing into each other until one vehicle remains running.

Several cars were still competing at the Tri-County Fair when one car collided with another.

Sheriff Roselles said the driver “came out of the turn he was unable to control the vehicle in any way.

“He ended up going over the top of the fence.”

“It was sad and scary,” Victoria Lopez told The Associated Press via Facebook. She and other witnesses said there are usually logs all around the arena at the Powell County Fairgrounds, but this year there was not one in the spot where the cars entered the arena.

Ms Lopez said one driver was unable to stop and his car “flew right through the fence where several people were at”.

She added: “They took off running, but not all made it.”

Meagan Beckett, who has training as a nurse, said she ran to help.

She saw responders performing CPR on some victims. “One gentleman had a neck brace on,” she said. “He seemed to be in a lot of pain.”

Two people were treated and released, three remained in a stable condition in hospital and one was flown to another hospital. Sheriff Roselles added that a one-year-old child was among the injured.

The driver was not among those taken to the hospital, Sheriff Roselles said.

The Butte-Silver Bow Fire Department tweeted their condolences over the death of a Powell County emergency medical technician at the demolition derby in Deer Lodge, which is about 80 miles south-east of Missoula.

Former promoter Tory Schutte of Genoa City, Wisconsin, estimated that about 5,000 derbies are held each year at county fairs nationwide. He said he was aware of fewer than a handful of deaths taking place since he became involved in the activity in the 1970s.

He said: “People get so excited about everything going on they just keep shifting toward the track.

“It’s the opposite of what our instincts would be.”

The derby was sponsored by the Powell County Chamber of Commerce.