Four teenagers have been taken to hospital with serious injuries after 16 passengers faced a four-and-a-half hour nightmare trapped on an Alton Towers rollercoaster following an accident.
The crash happened at around 2.09pm on the Staffordshire resort's 50mph Smiler ride after two carriages crashed on a low section of the track.
Two males and two female teenagers suffered serious leg injuries in the crash, while the other 12 occupants required triage.
An Alton Towers spokeswoman said: "I can confirm that the ride has now been fully evacuated and those with serious injuries have been taken to hospital.
"The evacuation was completed by 6.35pm."
The spokeswoman said she could not confirm reports that two of the casualties were taken to a hospital in Coventry and the other two to Stoke.
A ramp 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.
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.
Following the incident Alton Towers owner Merlin Entertainments was the biggest faller on the FTSE 100 after the crash, with its shares down 3%.
Visitors to Alton Towers reported on social media that the ride had broken down earlier today.
Lucy Farrugia tweeted: "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."
Sophie Underwood, who was waiting to board the ride, told the BBC: "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 in to 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.
Four air ambulances rushed to the scene together with four ambulances and several senior paramedic managers.
Staffordshire Fire and Rescue said it had sent four pumps, a rescue tender, rope rescue and an aerial ladder to the incident.
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."
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".
In July 2013 it 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.
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 in May 2013.
They were left dangling after the rollercoaster ground to a halt at a 50 degree angle.
Merlin Entertainments, which is based in Poole, Dorset, runs Alton Towers since buying out previous owner the Tussauds Group in May 2007.
It is the world's second-largest visitor attraction operator behind Disney and runs 105 attractions, 11 hotels and three holiday villages in 23 countries.
Ian Crabbe, Alton Towers divisional director, said: "May I first say that my colleagues and I are devastated by what has happened here today, and 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.
"Our concern is with and for them and for their safety, and all our thoughts and efforts are focused on them."
Mr Crabbe said that the park's first responder team were at the scene "in minutes" with the emergency services attending shortly afterwards.
He added: "A full investigation is under way, and representatives of the Health and Safety Executive are already on site and working with us."