Four people have been seriously hurt in a crash between two carriages on a rollercoaster ride at Alton Towers.
Sixteen people were in one carriage of the Smiler ride; the other was empty.
West Midlands Ambulance Service said two men, aged 18 and 27, and two women, aged 19 and 17, suffered "significant lower limb injuries". They were airlifted to trauma centres in Stoke and Coventry for treatment.
Alton Towers said all guests involved were evacuated by 18:35 BST.
A director described it as "the most serious incident" in the park's history.
The passengers were trapped for several hours on the ride, about 25ft (7.6m) above the ground.
Eyewitnesses reported hearing "a loud crash" when the crash happened at 14:09.
Ben Richardson said: "When the second carriage crashed people were screaming and shouting - even after it stopped. Everyone around the park ran over.
"The people looked significantly distressed. It was almost like a car crash, very full-on."
Alton Towers said first responders based at the park were on the scene "within minutes", quickly followed by the emergency services.
Hospital trauma team consultants were also brought in to help treat passengers.
Alton Towers has said the park will be closed on Wednesday "following the dreadful incident on The Smiler".
In a statement the theme park said: "All guests with pre-booked tickets, or those who arrive at the theme park, will have the choice of either having their tickets revalidated for an alternative date or a full refund."
Sophie Underwood, who was in the park at the time of the crash, said: "It wasn't very high but it was obviously high enough for them to cause some quite serious injuries to the people that were on the rollercoaster.
"They literally had come back off and round the back of a loop, and straight into another empty coach that had been stuck."
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.
The resort claims it features "a series of twisted psychological effects including optical illusions, blinding lights and near misses designed to mess with your mind".
There have been several accidents and incidents involving the £18m ride since it opened in May 2013.
History of problems
- A group of 16 journalists were left dangling on it at a steep angle as they tried out the rollercoaster before it opened to the public.
- The ride shut for four days in July 2013 after a piece fell off the track, and 48 people had to be rescued.
- In November 2013 it closed for five days after wheels fell off and hit four people in the front carriage. The injured people were looked after by park staff and did not need treatment by the ambulance service, a spokeswoman said at the time.
- Riders were also left stranded in the air when the ride ground to a halt at the top of a near vertical section 14 months ago.
Ellis Dyson, 23, who was in the queue for the ride, added: "The ride was delayed because of a technical fault for a while and then the ride came back on.
"They sent a carriage without any people on it first and then sent a carriage with people on and that was the one that crashed. The platform of the ride where we were vibrated and a massive loud crash."
Visitors reported on social media that the ride had broken down earlier in the day.
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."
Merlin Entertainments said there would be a "full investigation" and the Health and Safety Executive were already on site.
Ian Crabbe, divisional director at Alton Towers said the whole team at the park was "devastated" by the incident.
"Our thoughts and main concerns and focus are with injured people and the 16 people that were stuck on the train and their immediate families. That is our major concern," he said.
Merlin Entertainments was the biggest faller on the FTSE 100 after the crash, with shares down 3%.
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.
A help-line for concerned relatives has been set up by the Park which is 0800 230 0770.
Alton Towers is located near the village of Alton in Staffordshire and used to be a country estate.
It originally became a tourist attraction because of its gardens, but travelling fun fair rides were added to the grounds in the 1950s.
The installation of the Corkscrew rollercoaster in 1980 is regarded as a key point in the development of Alton Towers into a major theme park.
It now has some of the UK's best known rollercoasters - including Nemesis, Oblivion, Air and Rita - and attracts millions of visitors every year.