Alton Towers said the incident took place on a low section of the rollercoaster and involved one empty carriage and one containing 16 guests.
“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,” a statement said.
West Midlands Ambulance Service said that they were called to the resort at 2.09pm to reports that a moving carriage had collided with a stationary one.
A spokeswoman for the service said: “There are 16 patients on board The Smiler requiring triage, four of which have reported serious injuries.
“Ambulance staff and medics are working quickly with resort staff to gain access to the seriously injured.”
In a later statement the ambulance service said: “Four teenagers, two males and two females, suffered serious leg injuries in the incident and are receiving treatment as we speak.”
A platform has been built to allow a “safe and timely evacuation of guests”.
Alton Towers said there would be a full investigation of the incident, but that the priority at the moment was recovering guests.
The Smiler ride opened two years ago and is the world’s “first 14 looping rollercoaster” which combines “plunging 30 metre drops and extreme turns”.