Several children were injured in a serious crash on Highway 400 southbound after a school bus flipped upside down on the roadway early Saturday afternoon.

An Ontario Provincial Police officer who spoke with CP24, Const. Graham Williamson, says there were 34 people on the bus at the time of the crash. Twenty-one received minor injuries, while four others were more seriously injured.

The majority of the injured are children, says Williamson. None of the children suffered injuries that police consider life-threatening.

The vehicle rolled over on a ramp between the highway and Highway 9 while on the way to the Toronto area from a summer camp in Gravenhurst. The children were on their way to be reunited with their parents.

A CP24 caller reported seeing a number of ambulances and an ambulance bus heading to the scene, located at Highway 400 and Davis Drive.

The southbound lanes of the highway were closed for about three hours at Highway 88, near Bradford, but reopened before 4 p.m.