An air ambulance flies over the Smiler ride at Alton Towers in Alton, Britain June 2, 2015. Four teenagers were seriously hurt at one of Britain's biggest theme parks on Tuesday when two carriages collided on a rollercoaster ride, rescue workers said.  [Photo/Agencies]

LONDON - Four teenagers have been seriously injured as two carriages collided on the "Smiler" rollercoaster at Alton Towers, a famous theme park in Staffordshire on Tuesday afternoon.

Sixteen people were riding in one carriage on the attraction when it crashed into an empty stationary carriage, according to the West Midlands Ambulance Service (WMAS).

The four most severely injured teenagers included two male and two female. They have significant lower limb injuries. Two of them have gone to to the Royal Stoke University Hospital and two of them have gone to the University Hospitals Coventry and Warwickshire, said WMAS.

At least three people were still trapped on the ride, who were about 8 meters up in the air at an angle of about 45 degrees. Rescue men have built up a platform in order to reach them.

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."

Danny Simm, who was at the theme park when the accident happened, tweeted: "People unconscious, knocked out. Blood everywhere. It really was shocking. Air ambulances landing now, people still stuck."

The resort said their first aid staff were on the scene immediately to assist with the evacuation and the area has been closed to allow for access to emergency services vehicles.