Four people have been seriously injured in a collision between two carriages on Alton Towers amusement park's Smiler rollercoaster.
The collision involved 16 guests, with the carriages coming together on a low section of the track.
An air ambulance has arrived on the scene to deal with potential casualties.
The four passengers hurt in the crash are all teenagers who sustained serious leg injuries.
Yahoo News spoke to witness Kirsty Wild, 20, from Barnsley.
Kirsty, who has been going to Alton Towers with her mum for 15 years, said two empty carts on the ride's tracks had been put into operation while she was queuing for the ride, along with two carts full of visitors.
She said: "I heard a very big bang... then there was a lot of panic."
She said when she looked up at the ride one of the carts was swinging back up one of the loops of the rollercoaster from the impact of an apparent collision. "People on the two carts were crying", she said.
She added that a man was covered in blood, 'probably from a broken nose'.
Eyewitnesses to the incident at the theme park in Staffordshire described how they saw people with “crushed” faces.
Kitkatnic tweeted: “Can you let us know those people are okay, I saw it happen it was terrifying - that poor guy with his face smashed in!”
Rich_J_Boot added: “The cars on an angle at the bottom and some people were hanging to the side.”
Visitors to Alton Towers reported on social media that the ride had broken down earlier today.
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."
West Midlands Ambulance Service confirmed among those most seriously injured were two teenage boys and two teenage girls.
A spokeswoman added: 'Of the 16 occupants on board the ride, two teenage boys and two teenage girls, have sustained serious leg injuries.
'A platform has been build up in order for emergency services to reach the occupants on the ride who are approximately 25 feet up in the air at an angle of about 45 degrees.
'Our HART paramedics, doctors, Staffordshire Fire and Rescue Service and Alton Towers’ rope rescue staff are working at height to carefully release and treat the four seriously injured teenagers.
'The 12 other occupants on the ride are said to have suffered minor injuries and it’s hoped that they will be discharged from the scene.
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 Smiler opened in May 2013 and is billed as the world's first 14 loop rollercoaster.
It holds the official Guinness World Record for most loops in a rollercoaster, according to the Alton Towers website.