Alton Towers amusement park has been closed while an investigation continues into a "dreadful" accident on a rollercoaster which left four people with serious injuries.
The accident happened at around 2pm yesterday on the Staffordshire resort's 50mph Smiler ride when two carriages crashed on a low section of the track, leaving some passengers trapped for four-and-a-half hours.
Two men aged 27 and 18 and two women aged 19 and 17 suffered serious leg injuries in the crash.
The other 12 occupants - six men and six women - required medical treatment, including a man in his 20s who suffered neck and abdominal injuries.
Nick Varney, chief executive of Merlin Entertainments, which runs the park, said last night: "This has been a terrible incident and a devastating day for everyone here.
"I would like to express my sincerest regret and apology to everyone who suffered injury and distress and to their families.
"The safety of our visitors is our primary concern. The park will remain closed until we understand better the cause of this dreadful incident.
"I would like to thank the emergency services for their swift and effective response - they were outstanding.
"A full investigation is now under way and we will continue to work closely with the emergency services and the Health and Safety Executive."
The four people who suffered critical injuries were airlifted to major trauma centres after the 16 occupants were rescued from 25 feet up in the air at an angle of about 45 degrees.
A West Midlands Ambulance Service spokeswoman said: "All four were given advanced trauma care, pain relief and immobilisation and were each carefully extricated from the ride and onto the platform before being lowered to the ground.
"The 27-year-old male was then airlifted to University Hospital Coventry and Warwickshire whilst the others were flown to Royal Stoke University Hospital for further emergency treatment.
"The remaining 12 occupants, six women and six men, suffered less serious injuries.
"They were released one at a time over a time period of four hours and lowered to the ground in order for a further assessment of their condition.
"One of the 12, a male in his 20s was treated for neck and abdominal injury and was taken to the Royal Stoke University Hospital by land ambulance for further assessment and treatment."
The ordeal for some of the 16 occupants lasted more than four hours, with the evacuation not complete until 6.35pm.
A full investigation with the involvement of the Health and Safety Executive is under way at the park.
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.
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.
An Alton Towers spokeswoman said it is hoped the park will open again tomorrow. Customers with tickets for today can change them to another day or request a refund through the website.
Following the accident, Merlin Entertainments was the biggest faller on the FTSE 100, with its shares down 3%.
The firm, which is based in Poole, Dorset, has run Alton Towers since buying out previous owner the Tussauds Group in May 2007.