Of the 16 people involved in the incident, four have “significant lower limb injuries”, reports the Guardian’s Damian Gayle, who is at the scene.

A senior paramedic who spoke at the earlier press conference added: “Of the other 12 we now believe they will be walking wounded patients and the delay has just been extricating them from the cars.”

Of the four critically injured, two women in their mid twenties have gone to to the Royal Stoke University Hospital and two men have gone to the University Hospitals Coventry and Warwickshire.

Here is some aerial footage of the scene from earlier

We’re going to wrap up the liveblog now.

The Guardian’s Damian Gayle is at Alton Towers and says that three people were still on the suspended rise at 6.15pm.

He and other members of the media are being kept well away from the scene however and can’t actually see it. Here’s what he told me a few minutes ago:

At least one air ambulance helicopter is still at Alton Towers, where a press conference involving the emergency services has been under way.

We’ll bring your updates from that shortly.

Here’s a bird’s eye view of the scene, via an ITN helicopter, showing emergency vehicles at the scene.

An update on the injured. Sky News are reporting that the four teenagers with the most serious injuries are on their way to hospital and that their injuries are not life threatening.

A statement has been issued by Ian Crabbe, Alton Towers divisional director, who said:

May I first say that my colleagues and I are devastated by what has happened here today, and our only priority at this time is to work with the emergency services to get the 16 people affected off the ride safely, and most importantly those who are injured are being cared for.

Our concern is with and for them and for their safety, and all our thoughts and efforts are focused on them.”

Mr Crabbe said that the park’s first responder team were at the scene “in minutes” with the emergency services attending shortly afterwards.

He added: “A full investigation is under way, and representatives of the Health and Safety Executive are already on site and working with us.”

We’re waiting for a full update from West Midlands Ambulance Service but a spokesman has just confirmed the occupants of the Smiler are still stuck in the ride as rescue services work to release them safely.

Twitter user @laurenletitia13 captured the dramatic landing of air ambulance helicopters on film at Alton Towers earlier this afternoon.

West Midlands Ambulance Service, Staffordshire Fire, Staffordshire Police and Alton Towers Resort will be conducting a joint media briefing at Alton Towers Resort at 6.15pm.

We will bring you the latest updates.

West Midlands Ambulance Service has released a series of pictures of the scene at the Smiler collision.

Alton Towers owner Merlin Entertainments was the biggest faller at the close of the FTSE 100, with its shares down 3.46%.

The firm, which is based in Poole, Dorset, runs Alton Towers since buying out previous owner the Tussauds Group in May 2007.

With a market cap of £4.67 billion, it is the world’s second-largest visitor attraction operator behind Disney and runs 105 attractions, 11 hotels and three holiday villages in 23 countries.

The £18m rollercoaster is Alton Towers’ biggest ever investment, taking over eight months to build under tight secrecy. The length of its track is the equivalent of 10 football pitches and includes 14 loops, breaking the previous world record of 11 when it was unveiled in May 2013.

However, its glitzy media launch was overshadowed when it stalled at 100ft after a couple of runs.

An Alton Towers spokeswoman said at the time the problem had been caused by a delay at the ‘train station’ where passengers get on and off. This automatically triggered a failsafe which stopped carriages along the track.

It is not yet clear what caused this afternoon’s crash, although the ride certainly appears to have been experiencing problems before the collision at 2pm.

There has been a series of incidents on the Smiler since its opened in May 2013.

Sixteen people had to be rescued after it broke down during a press launch in its opening month.

In July 2013, 48 people were evacuated after witnesses saw a metal bar break away from the ride before landing near them, exposing a gap in the track and forcing another shutdown.

In November 2013, guide wheels fell off the ride and hit four people in the front carriage, once again forcing it to close.

Alton Towers has set up a helpline for concerned relatives on 0800 230 0770.

Outside Alton Towers, a visitor to the amusement park said she was told by staff that there was “technical issues” with the Smiler shortly before the crash happened.

The woman, who was not named by Sky News, said one of the test carriages “got stuck” but staff still sent another carriage full of people on to the ride.

“They were stuck at the top for 10 to 15 minutes as you first go up. Next minute, we were about to get on the next ride, and they’ve come straight down and crashed into that other one. We just heard this big almighty bang and screaming.”

She added that she saw a boy’s face “absolutely pouring with blood” and four girls screaming next to him. Another little boy on the ride was shouting “daddy, daddy”, she said.

Alton Towers owner Merlin Entertainments saw its share price plunge when it emerged the Smiler had been involved in a collision.

Stock in the FTSE 100 firm dropped 3.46% to 444.3p per share after opening this morning at 462.6p a share.

A number of Alton Town visitors have described experiencing problems on the Smiler rollercoaster hours before the crash.

Sophie Underwood told the BBC: “I think there were quite a few people that were angry because they’d told the staff that the coach before had stopped, that they hadn’t actually pulled through, so there were quite a lot of people making complaints to say they had said that the coach had stopped before it, but they ushered everybody back because there were people injured.”

James Gay, another visitor to the amusement park who saw the aftermath, questioned why the ride was still open after earlier technical glitches.

He tweeted: “People were stuck on The Smiler a couple of hours ago, they let the ride stay open and two carriages collide.

“One carriage empty and one full. One person looks badly injured at the front.

“Alton Towers, when are you closing this ride down for good? The ride had more than one problem today, I wish them all a recovery. It wasn’t a nice experience at all.”

He added: “It was awful to see, I wanted to climb down and do what I could to help but the response time from the professionals was very quick.”

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 Guiness 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”.

Here’s a promotional video from the amusement park, featuring a point-of-view perspective of riding the Smiler.

Alton Towers said a full investigation will be launched once the occupants of the ride are freed.

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.

West Midlands Ambulance Service have said the 16 occupants on the ride are stuck 25ft up in the air at a 45 degree angle.

An updated statement read:

Of the 16 occupants on board the ride, two teenage boys and two teenage girls, have sustained serious leg injuries.

A platform has been build up in order for emergency services to reach the occupants on the ride who are approximately 25 feet up in the air at an angle of about 45 degrees.

Our paramedics, doctors, Staffordshire Fire and Rescue Service and Alton Towers’ rope rescue staff are working at height to carefully release and treat the four seriously injured teenagers.

The 12 other occupants on the ride are said to have suffered minor injuries and it’s hoped that they will be discharged from the scene.

Once released from the ride, all four seriously injured teenagers will be airlifted to major trauma centres at University Hospital Coventry and Warwickshire and Royal Stoke University Hospital for further emergency treatment.

As the incident unfolded, a number of eyewitnesses shared images and described the events on Twitter.