Provincial police said five people were transported to hospital, two with life-threatening injuries, and one was re-routed to Sunnybrook Medical Centre in Toronto.
Aaron Lazarus of Lakeridge Health says police were going from car to car checking for injured people on the closed 10-kilometre stretch of highway about 80 kilometres east of Toronto.
Reports say dozens of vehicles were involved in the Friday afternoon collision, but police say they don't have a count. Firefighters say one driver had to be cut from his rig.
Police said they expected to reopen the eastbound lanes of Highway 401 between Newcastle and Newtonville around 10:30 p.m. ET and the westbound lanes a couple of hours later.
Police say Toronto Transit helped transport stranded motorists to a recreation centre in Newcastle, where Red Cross is on scene to provide assistance.
Const. Linda Wolf says police are looking into a blast of winter weather as a possible factor in the crashes.
Environment Canada had warned of snow squalls off Lake Ontario in the area.
"The weather is not ideal at this point," Wolf said.
Spokeswoman Vanessa Thomas said Metrolinx supplied one bus at the request of the provincial police to provide shelter for the occupants of vehicles involved in the crash.
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