Four teenagers have been seriously injured after two carriages on a rollercoaster collided at the Alton Towers theme park in central England.
The accident on Tuesday afternoon happened on The Smiler, an 80kmh ride featuring 14 loops.
Two male and two female teenagers suffered serious leg injuries in the crash, while another 12 people also needed medical attention.
A ramp was built to help emergency services reach the occupants of the ride, who were left eight metres up in the air at an angle of about 45 degrees.
Sophie Underwood, who was waiting to board the ride, told the BBC a carriage carrying passengers crashed into another empty one that was stationary at the top of the ride.
"It was quite scary. There was a big crash and as soon as everybody heard the crash everybody started walking back and leaving the ride," she said.
Danny Simm, a UK radio presenter who witnessed the aftermath of the accident, describing the scene on Twitter.
"People unconscious, knocked out. Blood everywhere. It really was shocking. Air ambulances landing now, people still stuck."
Ian Crabbe, Alton Towers divisional director, said he was devastated by the incident.
"Our only priority at this time is to work with the emergency services to get the 16 people affected off the ride safely, and most importantly those who are injured are being cared for.
He added that a full investigation was under way, and representatives of the Health and Safety Executive were already on site.
Alton Towers claims that The Smiler features "a series of twisted psychological effects including optical illusions, blinding lights and near-misses designed to mess with your mind".
