Opinion polls show Andrew Scheer's Conservatives with a real chance of defeating the ruling Liberals, whose campaign stumbled last month after images from years ago emerged showing Trudeau in blackface.
Trudeau has repeatedly apologized for the images.
The two-hour debate - the only one to be held in English, the language spoken by two-thirds of Canada's 38 million population - has traditionally been regarded as an important way of winning the minds of voters.
In one of the few truly memorable moments, Scheer noted the Liberal leader had not at first disclosed how many times he had put on dark makeup.
"He can't even remember how many times he put blackface on, because the fact of the matter is he's always wearing a mask," said Scheer.
"Mr. Trudeau, you are a phony and you are a fraud and you do not deserve to govern this country."