A BOLTON DJ has told of the shocking scenes at Alton Towers after a crash involving two rollercoaster carriages left four people seriously injured.

It happened on the amusement park's Smiler rollercoaster just before 2.10pm today.

Danny Simm, a presenter on Bolton FM, was enjoying a day out at the Staffordshire amusement park when the horror unfolded.

Mr Simm wrote on Twitter: "Big accident at Alton Towers, one train from The Smiler crashed into the other."

He added: "People unconscious on the ride with faces smashed in and blood everywhere... I think it will be closed for a while."

He also tweeted: "People unconscious, knocked out. Blood everywhere. It really was shocking. Air ambulances landing now, people still stuck."

The accident involved two carriages — one containing 16 passengers and the other empty — which came together on a low section of the track.

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."

The £18 million ride, opened in May 2013, has been closed down twice before because of technical issues.

It is billed as the world's first 14-loop rollercoaster and holds the official Guinness World Record for most loops, according to the Alton Towers website.

The resort claims that it features "a series of twisted psychological effects including optical illusions, blinding lights and near-misses designed to mess with your mind".

In July 2013 the Smiler was closed after reports that a bolt was seen to have fallen from the ride.

In November, the rollercoaster was closed after plastic guard wheels came loose and hit front row riders.

Today's accident involved a moving carriage carrying 16 passengers and a stationary carriage which was empty.

An Alton Towers spokeswoman said: "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."

The Midlands Air Ambulance carrying a trauma doctor on board, four ambulances and several senior paramedic managers arrived on the scene to deal with potential casualties.