The death toll "could certainly go higher but we won't know until we get into the vehicles and pull out the victims," provincial police Staff-Sgt. Doug Babbitt told a news conference.
Emergency crews planned to work into the night to remove the trapped bodies and possibly find others, provincial police said in a statement. Portable lights were brought in to help with the work.
The series of crashes occurred early Friday morning in the east- and west-bound lanes of Highway 401, 12 miles east of the Canada-U.S. border crossing at Windsor. There were at least five separate crashes, witnesses said, prompting a chain reaction of carnage involving at least 40 vehicles.
Flames roared from mangled tractor trailers as paramedics attended to the wounded lying in adjacent farm fields.
Kirk Walstedt heard tires squealing, horns honking and crashing noises from his home just east of the interchange where the crashes occurred.
"I could hear the smashes. Then you could hear them hit their brakes. And screams," he said. "It was just like a demolition derby."
Dave Phillips, who lives about a half a mile from the highway, said he heard a loud bang just after 8 a.m. and rushed to the scene.
He and 15 other people, mostly from vehicles involved in the crashes, lifted a car off one woman, and tried unsuccessfully to free another woman trapped between two burning vehicles.
"We could hear her screaming but we couldn't get to her," he said. "The fire just got too hot."
In the past, the road has been labeled a "killer highway" for its high number of fatal car crashes.
Recently, the Canadian Automobile Association said Highway 401 would be more forgiving of human error if it were widened to six lanes.