Four teenagers have suffered serious leg injuries in a collision between two carriages on Alton Towers amusement park's Smiler rollercoaster.
The accident, at around 2.09pm, involved a moving carriage with 16 passengers and an empty, stationary carriage which came together on a low section of the track.
A West Midlands Ambulance Service spokeswoman said that four teenagers - two males and two females - were seriously injured in the crash, while the other 12 occupants required triage.
The £18 million rollercoaster, which boasts a world record-breaking 14 loops, has been closed on two occasions because of safety concerns since opening two years ago.
Visitors to Alton Towers reported on social media that the ride had broken down earlier today.
An Alton Towers spokeswoman said: "The emergency services including the air ambulance are now in attendance and are assisting resort staff as they work to evacuate those 16 people still on the ride.
"We can confirm that four guests have sustained serious injuries, and they are being treated at the scene until they can be evacuated.
"There will be a full investigation once all the guests have been evacuated, which is our priority."
Three air ambulances attended the scene together with four ambulances and several senior paramedic managers.
A platform was built to help emergency services reach the occupants of the ride, who were approximately 25 feet up in the air at an angle of around 45 degrees.