Emergency crews were called to the scene at about 14:09 BST
Four people have been seriously injured in a crash between two carriages on a ride at Alton Towers in Staffordshire.
Sixteen people were on one carriage of the Smiler ride, the other was empty.
The resort said their first aid staff were on the scene and emergency crews were building a platform to help reach those on the ride.
The air ambulance is also in attendance following the collision that happened at about 14:00 BST.
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."
Merlin Entertainment, which runs the resort, said in a statement: "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. 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."