A car on the Smiler ride that was carrying 16 people at the Alton Towers amusement park slammed into a stationary car on Tuesday afternoon, the West Midlands Ambulance Service said.

Four of the people onboard “reported serious injuries,” the service said. It said paramedics, four ambulances and an air ambulance carrying a trauma doctor were on the scene.

Visitors to Alton Towers said the Smiler had been closed down earlier in the day because of technical problems, but had reopened before the crash.

Musician and radio broadcaster Danny Simm tweeted that he saw “people unconscious, knocked out. Blood everywhere. It really was shocking.”

The Smiler is billed as the world’s first roller coaster with 14 loops. Alton Towers says it features “a series of twisted psychological effects including optical illusions, blinding lights and near misses designed to mess with your mind”.

It opened in May 2013, two months later than scheduled, and has twice been shut down because of technical or mechanical problems.

Alton Towers, 150 miles (240 kilometers) north of London, is one of Britain’s most popular tourist attractions.