The westbound lanes of Highway 401 near Napanee, Ont., have reopened after a pileup involving more than 75 vehicles, but the eastbound lanes remain closed.
The collision scene covered a three-kilometre stretch of highway between Napanee and Deseronto.
The eastbound lanes are closed between Country Road 41 and Deseronto Road, east of Belleville, Ont.
Ambulances transported nine people to hospital, Ontario Provincial Police said. Three of those patients were later taken to Kingston General Hospital with serious injuries.
A bus was sent to the scene to transfer 19 people to an emergency shelter. Some of them had minor injuries but did not want to go to hospital.
Several regular and heavy tow trucks have been on scene throughout the day.
The eastbound lanes are expected to reopen Wednesday evening, OPP said.