Staff are currently evacuating the ride and area.
Eyewitness Ben Richardson told the BBC: “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.”
This is not the first time the rollercoaster has suffered mechanical failures.
In November it closed after several wheels came off a carriage while it was ascending the tracks and hit four passengers in the face.
In August it closed for five days due to a “technical issue” and in July it was shut for four days when a piece fell off the track and 48 people had to be rescued.
The 14-loop rollercoaster opened last May and includes a drop of 30m (98ft)