Five children and four adults, thought to include a lollipop lady, were taken to hospital after a car overturned near a primary school.
A black Audi overturned near a pelican crossing and landed on its roof on the pavement, sandwiched between a green car and a line of trees, near Rhoose Primary School in the Vale of Glamorgan.
Emergency services described the "major incident" as "very serious", but it is understood there were no deaths. A lollipop stick lay on the ground at the scene and various pieces of clothing were strewn across the pavement.
The nine injured were taken to two different hospitals following the incident, a Welsh Ambulance Service spokesman said.
Four children with head and/or abdomen injuries and two adults with fractures were taken to the University Hospital of Wales in Cardiff, while two adults and another child, all with minor injuries, were taken to the Princess of Wales Hospital, he added.
The spokesman said the incident was "declared a major incident due to the number of casualties involved and their injuries".
Two rapid response vehicles, four emergency ambulances, a basics doctor, the Hazardous Area Response Team (Hart) and the Helimed helicopter were sent to the scene, he added.
The Wales Air Ambulance also sent an air crew to the incident in Fontygary Road, Rhoose, which happened as children were arriving at school. Its crew "flew through adverse weather conditions to make it to the scene" and treated three patients, but did not take any patients to hospital, a spokesman said. "Adverse weather continued to close in and the patients were assessed as suitable to travel by road," he added.
Ian Morris, divisional manager for surgery at University Hospital of Wales in Cardiff, said: "The emergency unit at University Hospital of Wales is treating adults and children involved in the incident in Rhoose. The unit has specialist procedures in place for dealing with serious incidents and those have been activated to care for those involved. The priority now is treating those involved and supporting their families."
The 61-year-old driver of the overturned car is assisting police with the investigation into the collision which is not thought to have been deliberate.