Emergency services have been scrambled to a car park in Sheffield city centre.
At least seven police cars, two police vans, three fire engines and two ambulances are on the scene at the Q Park on Durham Road, near the University of Sheffield students' union building, according to a witness.
He said police officers and other emergency crews arrived at the scene at around 4.45pm and rushed into the car park.
Police said three children and a woman had been taken to hospital, with one of the children having sustained serious injuries.
A witness has said vehicles are now being let out of the car park and ambulances and fire engines are heading off but road closures remain in place.