The six major party leaders squared off in a debate Monday that saw Liberal Leader Justin Trudeau and Conservative Leader Andrew Scheer taunt each other as they jockeyed for the front-runner position.

Scheer branded his Liberal opponent as a mask-wearing "phoney" and a "fraud" who has lost the moral authority to govern.

Trudeau, in turn, said Scheer doesn't believe in climate change, has no plan for the environment and promotes an agenda that disproportionately benefits the wealthiest Canadians.

NDP Leader Jagmeet Singh tried to stay above the fray with a positive approach as the Liberal and Conservative leaders slugged it out. Green Party Leader Elizabeth May courted climate-minded voters disaffected with the other parties.

Bloc Québécois Leader Yves-François Blanchet stuck to the separatist script, criticizing the federalist parties for ignoring the unique needs of Quebecers while directing most of his attacks at Scheer and that leader's pointed criticism of SNC-Lavalin during the scandal over prosecutorial independence.

Trying to peel away right-wing votes from the Tories, People's Party Leader Maxime Bernier called Scheer a half-hearted conservative who doesn't have the courage to end the "dairy cartel" and supply-managed farm sectors. Bernier also pushed his plan to slash immigration levels to better integrate newcomers.

'You are a phoney'

Scheer started the debate on the attack, focusing his energies on his main opponent in this election campaign: Trudeau.

Ignoring the first audience question on foreign affairs, the Conservative leader said Canadians simply can't trust Trudeau after a series of missteps including the blackface photos and the SNC-Lavalin affair that claimed two of his top female cabinet ministers.

"He's very good at pretending things. 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 reconciliation mask and then fires the attorney general, the first one of Indigenous background. He puts on a feminist mask and then fires two strong female employees for not going along with his corruption," Scheer said.

Conservative Leader Andrew Scheer says Liberal Leader Justin Trudeau is always wearing a mask at the outset of the English-language debate in Gatineau, Que. 0:35

"He puts on a middle-class mask and then raises taxes on middle-class Canadians. Mr. Trudeau you are a phoney and you are a fraud and you do not deserve to govern this country," Scheer said.

Trudeau did not have a chance to respond to the attack as the moderator moved along to the next question.

Scheer invoked the SNC-Lavalin scandal later in the debate during a segment on Indigenous issues. Trudeau said the Conservatives cannot be trusted to properly consult Indigenous people when weighing whether to approve a major natural resources project like the Trans Mountain expansion.

Scheer said he'll take no lessons from Trudeau on Indigenous issues after he kicked Jody Wilson-Raybould, the first justice minister of First Nations ancestry, out of the Liberal caucus over the SNC-Lavalin matter.

Justin Trudeau says he'll move forward with energy projects with respect for indigenous people, while Andrew Scheer says he has 'nothing to learn' from Trudeau, who 'fired' Canada's first indigenous justice minister. 1:29

On SNC-Lavalin, Trudeau said he would not apologize for standing up for Canadian jobs.

Amid the partisan potshots, Singh said Canadians are looking for positive leadership from Ottawa.

"That's what a leader does and a leader works for the people who need help, not helping those at the very top which we've seen with governments in Ottawa for far too long," Singh said.

Bernier promotes immigration cut

A key plank of the People's Party platform is a promise to cut immigration levels significantly, something that has drawn the ire of the other parties as an intolerant plan that will damage the economy of an aging country.

Bernier insists that an immigration cut is the best way to integrate newcomers, but the other parties have said that's cover for a platform designed to appeal to racist elements in Canadian society.

"We don't want our country to be like other countries in Europe where they have a huge difficulty to integrate their immigrants," Bernier said.

'Your role on this stage seems to be to say publicly what Mr. Scheer thinks privately,' Liberal Leader Justin Trudeau says to People's Party Leader Maxime Bernier during the English-language debate in Gatineau, Que. 1:07

Bernier said few Canadians want to see many more immigrants. "Only six per cent of Canadians want more immigration. Only six per cent. So when you don't want to have a debate about that you're not in line with the population. Let's just have an honest debate on that subject, OK?" he said, citing an unnamed poll on immigration.

Trudeau accused Scheer of harbouring the same anti-immigrant sentiments as his one-time Conservative leadership foe.

"Your role on this stage seems to be to say publicly what Mr. Scheer thinks privately," Trudeau said to Bernier.

Scheer said the opposite is true adding he believes in a "fair, orderly and compassionate" immigration system.

"That is the difference between Mr. Bernier and myself on this issue. We believe in making Canada stronger by welcoming people, adding to our country and celebrating the things that have made us great as a nation," Scheer said.

Trudeau challenges Singh on Bill 21

The squabbles over what the federal government should do about Quebec's Bill 21, the province's secularism law, continued Monday with Blanchet calling on the other leaders to stay out of the province's internal affairs.

The law prohibits civil servants from wearing religious garb on the job. While denounced by many in English Canada as a discriminatory law, Quebec nationalists have heralded the legislation as the best way to protect the religious neutrality of the state.

Trudeau has said a federal government led by him would consider intervening in an ongoing citizen-led court challenge to the law. He called on Singh to do the same.

NDP Leader Jagmeet Singh speaks about his battle against discrimination after Liberal Leader Justin Trudeau challenges him to say he'll intervene on Quebec's secularism law. 1:15

So far, Singh has been coy about his intentions with the contentious law, which is supported by a solid majority of francophone Quebecers.

Singh has said he opposes discrimination of any sort and, as a turban-wearing Sikh, he understands first-hand how difficult it can be for minorities, but he also refused to challenge the law's constitutionality.

"Mr. Singh, you have spoken very eloquently about discrimination and fought against it all your life. And that's why it's so surprising to have heard you say, like every other leader on this stage, a federal government under you would not intervene on the question of Bill 21," Trudeau said.

"Yes, it's awkward politically because it's very popular but I am the only one on stage who has said, 'Yes, a federal government might have to intervene,'" Trudeau said.

In a post-debate scrum, Singh said he'd stay on the sidelines for now. "I think it's very important that we don't interfere with the court challenge, and I made that really clear," Singh said.

'I'm pro-life'

Trudeau again tried to pin Scheer down on social issues, namely his stance on abortion and a woman's right to choose.

Scheer, a long-standing social conservative, said he is anti-abortion but a Tory government led by him wouldn't reopen the debate to limit a woman's access to the service.

"Like millions of Canadians, I am personally pro-life. It is OK in this country to have a difference of opinion," Scheer said to Trudeau.

Green Party Leader Elizabeth May notes it has been male leaders mostly debating abortion in this election and the 'hard-earned rights' women. 2:13

Singh jumped in to say a man should never tell a woman what to do with her own body — and beyond accepting abortion, Singh said it should be more readily available to women who want it.

"I'm someone that believes in, firmly and unequivocally, the right of women to choose and to build more access to abortion services," Singh said.

May said, as the only woman on stage, she'd never tolerate any move to curtail access.

"We will never allow a single inch of retreat from the hard-earned rights of women in this country — not one inch," May said. The Green Party dumped a candidate Monday who professed anti-abortion views.

On climate, 'A slogan is not a plan': Trudeau says

One of the central themes of the campaign to this point is the climate change fight. The left-of-centre parties have tried to court environment-minded voters with competing plans to drive down greenhouse gas emissions to help Canada meet its Paris targets and stop global warming.

May has called for the most aggressive action on this file, proposing to double the country's current target — from 30 per cent to 60 cent per cent below 2005 levels by 2030 — by closing down much of the county's natural resources sector.

No new pipelines, or coal, oil or gas drilling or mining, including offshore wells, would be approved under a Green government. The party is also promising to ban all gas-powered — internal combustion engine — cars by 2030.

"We're up against a real climate emergency," May said. "Greta Thunberg is right. The house is on fire and the grown-ups need to stand up and say, 'Kids get to safety. We've got this.'"

May was critical of Trudeau maintaining levels previously set by the former Conservative government, while adding a committed climate fighter doesn't buy a pipeline like Trans Mountain.

"A slogan is not a plan, Ms. May," Trudeau retorted.

Green Leader Elizabeth May and Liberal Leader Justin Trudeau have a spirited exchange on climate change plans and emissions targets. 1:23

Trudeau said he's serious about climate action but what May is proposing could never actually be accomplished on such a short timeline.

On the environment, Scheer touted his green home retrofit tax credit. He said Canadians simply don't trust Trudeau's plan, which relies on a carbon tax to help offset emissions.

"The choice tonight is very clear between two parties that have very different views on climate change. Mr. Scheer wants to rip up the only serious plan on climate change Canada has ever had," said Trudeau.

"You do not need to choose between Mr. Delay and Mr. Deny," Singh said as Trudeau and Scheer discussed the merits of a carbon tax.