Witnesses told broadcaster SVT the ride had partly derailed and a helicopter, ambulances and fire engines were seen arriving.

Police confirmed seven children and adults had been injured at the Grona Lund theme park and were being treated at hospital. They are still assessing the extent of the damage.

"We are emptying the park and we've started our investigation," a police spokesperson said.

A black banner on Grona Lund's website said: "A serious accident has taken place on the Jetline. We are working with the rescue services. The park is closed for now."

Cornelia, who had just been on the ride with her son, told Sweden's TV4 that people started running.

"There was a lot of screaming. No one knew what was going on," she said.

Sweden's Expessen quoted a police officer as saying several people had fallen off the ride and were found on the floor, while several more were left stuck.

Jenny Lagerstedt, who was visiting the park with family, said she heard a metallic noise and saw the track shaking.

"My husband saw a rollercoaster car with people in it falling to the ground," she told SVT.

A witness queuing nearby told Expessen he saw the rollercoaster's wheels starting to come off.

Jetline reaches 56mph (90kmh) and is 30m (98ft) high, carries more than a million people per year, and costs 90 kronor to ride (£6.60), according to the park's website.

It also says all rides are checked daily for safety and that Dekra, a state control body, inspects them annually.