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UPDATE 2 -Canada opposition leader slams Trudeau as a fraud in key election debate

David Ljunggren and Kelsey Johnson

debate@ (Releads with details of debate)

OTTAWA, Oct 7 (Reuters) - Andrew Scheer, Canada's main opposition leader, on Monday kicked off a key televised election debate by lashing Prime Minister Justin Trudeau as "a phony and a fraud" who did not deserve to win an Oct. 21 election.

Polls show Scheer's Conservatives have a real chance of defeating the ruling Liberals, whose campaign stumbled last month after images 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.

Scheer, asked to comment on foreign policy, said Trudeau was only pretending to stand up for Canada and noted the Liberal leader had not at first revealed how many times he had put on dark make-up.

"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 ... he puts on a middle-class mask and then raises taxes on middle-class Canadians," said Scheer.

"Mr. Trudeau, you are a phony and you are a fraud and you do not deserve to govern this country," continued Scheer, who is promising to cut taxes and make life more affordable.

The debate grouped Trudeau, Scheer and the leader of four other parties, including Maxime Bernier of the right-wing People's Party of Canada. Bernier said he would slash immigration if elected.

"Mr Bernier, your role on this stage tonight seems to be to say publicly what Mr. Scheer thinks privately," said Trudeau, who accuses Scheer of harboring an extreme right-wing agenda.

A Nanos Research poll for CTV and the Globe and Mail on Monday put the Liberals at 34.3% and the Conservatives at 33.4%, in contrast to surveys that showed Trudeau falling behind after the blackface scandal broke on Sept. 18.

Trudeau has gained momentum since Scheer stumbled through a French-language debate on private channel TVA on Wednesday. During that event Trudeau went on the offensive against Scheer, often interrupting his rival.

French is the main language in the populous province of Quebec, which accounts for 78 of the 338 seats in the House of Commons. The leaders will also face off in a second French debate on Thursday.

Scheer is also facing questions about why he did not reveal he has dual U.S. citizenship. (Additional reporting by Steve Scherer in Toronto; Editing by Dan Grebler and Sandra Maler)