Images posted on social media showed a mangled carriage and a number of people trapped upside down in their seats in the Tsunami ride at M&D's amusement park near Glasgow.
The ride had been full when it derailed, according to eyewitnesses at the park, which was evacuated after the accident according to its website.
"Like a horror film worse thing ever heard and seen, can't get over this," Katie Burns, who had just ridden the rollercoaster and was walking past it when the crash occurred, wrote on Facebook.
Lanarkshire Police initially said two adults and nine children had been injured, but later revised the number of children hurt to eight.
Chief Inspector David Bruce said a series of five gondolas on the inverted rollercoaster -- meaning that the gondolas hang from the rails rather than run on top of them -- had detached, "struck the superstructure and then struck the ground".
"They fell less than 20 feet. It would appear that they have been coming round a bend and at that point it has detached .. I don't know what speed it would have been travelling but it was operating and rollercoasters do travel at speed."
Six ambulance crews, a specialist operation team and emergency doctors were dispatched to the scene, said a spokesman for the Scottish Ambulance Service.
