Four teen​a​gers were seriously injured on Tuesday when cars on one of England’s most famous roller coasters collided and stranded other passengers 25 feet above the ground, officials said.

The crash at ​the ​Alton Towers amusement park, about 150 miles north of London, happened a little after 2 p.m. local time, leaving a horrific scene for paramedics and rescuers.

“People unconscious, knocked out. Blood everywhere,” park visitor Danny Smith tweeted. “It was really shocking.”

One car on the Smiler ride, carrying 16 people, slammed into another car that was empty and stationary, according to the West Midlands Ambulance Service.

Two teenage boys and two teenage girls suffered serious leg injuries and were taken to hospitals by helicopter, the ambulance service reported.

Witness Ben Richardson told the Daily Mail the crashing roller coaster must have been going about 50 mph when it collided with the other car.

“I saw it all, there was a huge bump and when they came together, they both crumpled because it was metal-on-metal at a high speed,” Richardson said.

“No one got thrown out, it was too far to see injuries but definitely they would have all had very bad whiplash at least. The passengers were screaming and people nearby were running over.

“The Alton Towers staff quickly moved people away from the area so we haven’t been able to go anywhere near it since.”

Richardson described the panicked screams of passengers.

“When the second carriage crashed, people were screaming and shouting — even after it stopped. Everyone around the park ran over,” Richardson told the BBC.

“The people looked significantly distressed. It was almost like a car crash, very full-on.”

A platform was built to help paramedics reach riders, trapped and dangling at a 45-degree angle.

The Smiler ride had been closed earlier in the day for technical problems but had reopened before the crash.

Park visitor Thomas Whelton was briefly trapped on board the Smiler three hours earlier but was quickly picked up by a rescue car.

“When we got stuck on the Smiler … we were at the bottom of the first hill and they sent a train down behind us, which I thought was a bit strange,” Whelton said.

“We went about halfway across the park and we heard a big bang and we didn’t know what it was.”

The Smiler opened in May 2013 and the park boasts that it’s the world’s first 14-loop roller coaster.

It has the Guinness World Record for most loops in a roller coaster, the Alton Towers website said.

The multiple loops cause “a series of twisted psychological effects including optical illusions, blinding lights and near misses designed to mess with your mind.”

After the crash, Alton Towers said there would be “a full investigation once all the guests have been evacuated, which is our priority.”

With Post wires