The southbound lanes of Highway 400 have reopened south of Highway 89 after being blocked for several hours following a serious three-vehicle collision in Innisfil, Ont. Monday morning.

Ontario Provincial Police blocked off the highway and the ramps from Highway 89 to 400 from 5 a.m. until approximately 8:30 a.m. as they cleaned debris and investigated the cause of the crash.

Police told CP24 the first vehicle spun out and broadsided a second vehicle, which was then rear-ended by a third vehicle.

At least two people were transported to hospital by ambulance, though the extent of their injuries is unknown.

An Ornge air ambulance landed at the scene when multiple injuries were reported in the crash, but none of the patients needed air transport, an Ornge spokesperson said.

The highway shutdown caused commuter delays as alternate routes became clogged by detouring vehicles.

South Simcoe Police reported several minor collisions on the detouring roads, including Highway 27, and 5th, 10th and 11th Sideroads.

"Every southbound route now has a collision," police said in a message posted on Twitter.