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In a statement, Alton Towers said: "There has been an incident on the Smiler this afternoon involving two carriages coming together on a low section of the track. One of the carriages was empty and the other had 16 guests in.
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.
Eyewitness 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."
"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.
"There was a big crash, and I think there were quite a few people that were angry cos they said that they had told the staff that the coach before had stopped but they hadn't actually called through."
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.
It closed again in August 2013 for five days due to a "technical issue".
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.