Fire crews arrived after midnight at row of commercial units — buildings described by fire officials as the original site of Cooksville in the 1800s.
The fire is thought to have started in a grocery store and spread to the other units. At one point, a wall of one of the buildings collapsed.
Three ladder units are on the scene as crews fight the blaze in the bitter cold. No injuries have been reported.
Dundas Street East is closed east of Hurontario Street.
Fire crews also responded overnight to a two-alarm house fire near Bayview and Moore Avenues.