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.
Emergency services built a platform to help people exit the ride.
Visitors to Alton Towers reported on social media that the ride had broken down earlier on Tuesday.
Lucy Farrugia said: "Smiler broke down when I was on it this morning and now it's crashed. Hope everyone on it is OK, saw the air ambulance arrive."
Danny Simm, a musician and songwriter who is also a radio presenter on 96.5 Bolton FM, who witnessed the aftermath of the accident, said that a number of people appeared to be badly injured.
He tweeted: "People unconscious, knocked out. Blood everywhere. It really was shocking. Air ambulances landing now, people still stuck.
The £18 million ride, opened in May 2013, has been closed down twice before because of technical issues.
It is billed as the world's first 14-loop rollercoaster and holds the official Guiness World Record for most loops, according to the Alton Towers website.
The resort claims that it features "a series of twisted psychological effects including optical illusions, blinding lights and near-misses designed to mess with your mind".
In July 2013 the Smiler was closed after reports that a bolt was seen to have fallen from the ride.
And in November that year the rollercoaster was closed after plastic guard wheels came loose and hit front row riders.
Today's accident involved a moving carriage carrying 16 passengers and a stationary carriage which was empty.
An Alton Towers spokeswoman said: "The resort's fully-qualified first responders were on the scene immediately to assist with the evacuation which is ongoing, and the area has been closed to allow for access to emergency services vehicles.
"To ensure the safe and timely evacuation of guests, a platform is being constructed. This will take a few minutes but will enable us to proceed more quickly.
"There will be a full investigation once we have recovered the guests, who are our priority."
The Midlands Air Ambulance carrying a trauma doctor on board, four ambulances and several senior paramedic managers have arrived on the scene to deal with potential casualties.
Staffordshire Fire and Rescue said it had sent four pumps, a rescue tender, rope rescue and an aerial ladder to the incident.
Sophie Underwood, who was waiting to board the ride, told the BBC what she saw and that the ride had been experiencing issues earlier in the day.
She said: "They had made quite a few announcements to say there were technical difficulties. They were sending coaches around with nobody on them.
"And then they said they had sorted it out so they decided to put people on the coach."
Ms Underwood described a carriage carrying passengers crashing into another 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.
Ms Underwood added that from where she had been, she could see people had been injured.
Witness Ben Richardson, who had been on the ride earlier in the day, told the broadcaster the empty carriage testing the system was static when the one carrying passengers was sent along the rollercoaster at "around 50mph".
He said: "On the second carriage people were screaming and shouting. Even after it had stopped there was screaming and shouting. Everyone was shocked and no-one knew what to do. It was a bit erratic and there was quickly a large crowd there."
Mr Richardson continued: "When the first carriage stopped and was completely static, the carriage they had got passengers on was at the top of the ride. At that point they could have unloaded the carriage and gotten people away safely.
"Instead they send the cart around, whether that was computer error or human error, I don't know. But common sense would say get the passengers off while you can."
He described a "huge bang" and said the scene looked like that of a "car crash".
In another previous scare, 16 journalists were left stranded on the Smiler for around 30 minutes during a preview ride before it opened to the public.
They were left dangling after the 50mph rollercoaster ground to a halt at a 50 degree angle.
James Gay, another visitor to the amusement park who saw the aftermath, questioned why the ride was still open after it had been hit by technical glitches throughout the day.
He tweeted: "People were stuck on The Smiler a couple of hours ago, they let the ride stay open and two carriages collide.
"One carriage empty and one full. One person looks badly injured at the front.
"Alton Towers, when are you closing this ride down for good? The ride had more than one problem today, I wish them all a recovery. It wasn't a nice experience at all."
He added: "It was awful to see, I wanted to climb down and do what I could to help but the response time from the professionals was very quick."
A West Midlands Ambulance Service spokeswoman said: "Four teenagers, two males and two females, suffered serious leg injuries in the incident and are receiving treatment as we speak.
"The other twelve occupants of the ride are not thought to be seriously injured. We still have a number of resources at the scene."