"There were maybe seven or eight individuals, mostly of younger age between maybe 10 and 15 years old that were actually trapped inside the rollercoaster lying on the ground. The harnesses were still on as well so there was absolutely no movement."
He said he thought the rollercoaster had slipped off the tracks as it came around a corner but did not fall from the highest point.
A Police Scotland spokesman said: "Emergency services are responding to an ongoing incident at M&Ds theme park in Motherwell and further updates will be given in due course."
A spokesman for the Scottish Ambulance Service said six ambulance crews, a specialist operation team and emergency doctors were on the scene.
He said: "I can confirm that we are attending an incident at M&Ds theme park in Motherwell."
According to the M&D's website, the Tsunami rollercoaster can go at speeds of up to 40 miles per hour through corkscrew turns and loops on a 650km track.
Children under 10 are not allowed to use the ride, and under 14s must be accompanied by a paying adult.
A spokeswoman for M&D's said: "A serious incident occurred at M&D's today involving the Tsunami rollercoaster.
"We have all emergency services on site to assist. The theme park is closed until further notice."
