According to the West Midlands Ambulance Service, there were 16 people riding on the Smiler roller coaster when the crash happened earlier today.
It is believed that they crashed into a stationary carriage that was empty at the time.
The park's first aid team were the first on the secene, and an air ambulance was quickly called, arriving with a trauma doctor on board.
A West Midlands Ambulance Service spokeswoman 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 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.
"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."
Eyewitness Ben Richardson told BBC News: "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."
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.