A freight truck struck their van at an intersection outside Caledon, about 25 kilometres (15 miles) north of Toronto Saturday evening, police said.
Dozens of emergency workers spent more than an hour trying to free the victims from the carnage, according to local reports.
Four people, including a 13-year-old boy and a 76-year-old woman, died at the scene, Ontario Provincial Police Constable Linda Kennedy told AFP.
Six people, including the tourists' Canadian driver, a 77-year-old man and three children aged six to 13 years, were rushed to hospital in ambulances and aboard two helicopters, where they remain in serious condition, she said.
Their names have not been released.
The driver of the tractor-trailer was not injured in the collision.

Copyright AFP (Agence France-Presse), 2006