Mulcair was referring to comments Harper recently made in Saskatoon where he was questioned at a meeting of rural municipal leaders about the government's priorities in dealing with firearms.
"It's a tool that many people use in their lives, obviously in their livelihoods," the prime minister told the Saskatchewan Association of Rural Municipalities.
"My wife's from a rural area, gun ownership wasn't just for the farm, it was also for a certain level of security when you're ways away from police, immediate police assistance.
"But also it's something people use for recreation and the vast, vast majority do so safely and, as you know, in many parts of the country, (it's) an important business."
In Montreal on Tuesday, Mulcair accused Harper of "using the firearms dossier to divide Canadians."
"This time it's an attempt to divide the rural world from the world that lives in cities," he said.
The NDP leader added he was "quite astonished" to hear the prime minister tell people to use their own weapons to protect themselves.
"Mr. Harper conducted his last campaign on the firearms registry and he's trying to stir up passions again," Mulcair added.
Harper made his comments while being questioned about the government's plans after dismantling what the moderator called "the ineffective and wasteful long-gun registry."
The prime minister went into detail defending Bill C-42, the so-called Common Sense Firearms Licensing Act currently before the House of Commons.
"We're obviously trying to make the rules and life simpler for conscientious law-abiding gun owners which represent the vast majority of the millions of people who have firearms in this country," Harper said.
Asked for his reaction to Harper's remarks, Quebec Premier Philippe Couillard said Tuesday a country with an abundance of firearms in circulation is not a country in which a majority of Canadians and Quebecers want to live.
"The fewer firearms in circulation in the country, the better," said Couillard, adding he hunts small game such as partridges.
Jean-Marc Fournier, the minister responsible for Canadian intergovernmental affairs, said Tuesday he was very surprised by Harper's declaration.
"My vision of Canada is certainly not to say that Canadians and Quebecers must be encouraged to carry out vigilante justice," he told reporters."On the contrary, when it's a question of protecting the public, it reverts back to different governments to improve their services.
"Making that declaration practically makes it legitimate for people to carry out justice themselves and even get access to a firearm to do it, which doesn't correspond to our system of law, our system of justice and our system of protection of the public."
Fournier said he hopes Harper will come out and say his comments may have been misinterpreted.
The province is waiting for the Supreme Court of Canada to hear a Quebec government appeal to preserve data from the long-gun registry, which was scrapped by the Harper government in 2012.
Quebec wants the data to support its own registry, saying it would cost too much to start a new one.
The province asked the top court to hear the case after losing at the Quebec Court of Appeal, which said it had no right to the data.
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