A new £18m roller coaster at Alton Towers has closed after wheels fell off and hit four people in the front carriage.
A spokeswoman said some "small guide wheels" came off The Smiler as it was being pulled up an incline.
Kevin Hayes-Heath, who was in the carriage with his partner Wayne, said the incident was "pretty frightening".
Debris hit him on the shoulder and his partner in the face. A teenage girl and her father were also hit, he said.
"You are at the highest point of the ride and you've got all these loops to do and you just think that the carriage is going to come off," Mr Hayes-Heath said.
"We were shouting at them to stop the ride because they were about to send another party off."
Alton Towers said the ride completed its circuit and no evacuation was required.
A spokeswoman confirmed that plastic guide wheels were flicked onto a teenager and three adults in the front row of the train.
The rest of the guide wheels were caught by safety netting, she added.
The injured people were looked after by park staff and did not need treatment by the ambulance service, the spokeswoman said.
The park said an investigation would be carried out into Saturday's incident to decide when the ride could re-open.
Safety 'absolute priority'
The 14-loop Smiler was shut for four days on 21 July after a piece fell off the track, and 48 people had to be rescued.
It closed again in August for five days due to a "technical issue".
The ride, which opened in May, includes a drop of 30m (98ft).
The spokeswoman said: "We have very stringent safety procedures on all of our rides including CCTV so the ride was stopped immediately by the rides team.
"All of our rides and attractions are expected to meet extremely high standards, technologically, mechanically and in terms of the end experience, and a full investigation is on going before we determine whether to re-open the ride.
"We are sorry for any disappointment the ride's temporary closure caused our visitors but the health and safety of all of our visitors is our absolute priority."