Oakwood theme park closed early following an accident on the Treetops rollercoaster on Sunday.
Eyewitness Harriet Lloyd said that a man had been “thrown out” of the ride’s train.
Speaking to the BBC, Ms Lloyd said: “It looked like the last few carriages were loose and wobbling. He’d been thrown out [and] other passengers were holding the carriage because it had become a bit loose.”
The Wales Ambulance service said they were called to the Welsh theme park at 13:00 and remained on site for several hours.
An air ambulance is understood to have flown one casualty to hospital. No further details have been released by the emergency services.
The theme park said shortly before 17:00: “As a result of an incident at Oakwood today we have closed the park to the public with immediate effect.”
Oakwood was due to remain open until 19:00 for Halloween events. A reporter for WalesOnline said that the car park was virtually empty by 15:00.
The Health and Safety Executive (HSE) has begun an investigation into the incident, with the Treetops ride almost certainly out of action for at least the rest of this season.