Open this photo in gallery Teen activist Greta Thunberg joins a climate strike march in Montreal on Friday. ANDREJ IVANOV/Reuters

Three of the main federal political party leaders participated in the climate strikes across the country Friday, throwing their support behind the call for more action by marching with Canadians, while Andrew Scheer stayed away from the demonstrations.

Liberal Leader Justin Trudeau met with Swedish climate activist Greta Thunberg ahead of Montreal’s march where thousands, many of them young people, took to the streets to call on world leaders to take action on climate change. Mr. Trudeau called the teenage activist “the voice of a generation," when speaking to reporters.

Scheer, who was in Montreal on Wednesday and Thursday, flew to Vancouver with his team Friday morning where he made an announcement, but skipped the demonstration taking place nearby.

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Speaking near a transit station, Mr. Scheer vowed that a Conservative government would replace British Columbia’s George Massey Tunnel and work on other projects to help reduce congestion and improve commute times.

When asked about his absence at the climate marches, Mr. Scheer said it is “always encouraging” to see so many people, especially young people, show their concern for the environment.

“It is why we have many candidates and members of Parliament who are there,” he said.

Canadians who are protesting see that Mr. Trudeau’s environmental plan is failing, he added, noting that the Liberal Leader is “protesting his own government’s record on the environment."

Green Party Leader Elizabeth May also attended the march in Montreal, while NDP Leader Jagmeet Singh marched in Victoria.

Although Mr. Trudeau used the climate strikes to highlight his record on the environment, he was met with protest over his government’s purchase of the Trans Mountain pipeline project. As he walked alongside Quebec Liberal candidates, his wife and two of his oldest kids, some protesters criticized him for purchasing the Trans Mountain pipeline, while others were excited and tried to take pictures.

The loudest chants against him came from a group shouting: “No pipeline, no pipeline.” In response, Mr. Trudeau and his candidates chanted in French: “We’re moving forward for the planet.”

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In June, Ms. Thunberg tweeted that the pipeline purchase was “shameful." When asked by reporters about her tweet, Mr. Trudeau said his response to her and the protesters is that the Liberals would “hit our 2030 targets” while also ensuring the country gets a “better price for our oil resources,” with the profits going toward combating climate change.

Mr. Trudeau also announced Friday that a re-elected Liberal government would plant two billion trees over 10 years, creating 3,500 seasonal jobs each year.

The tree-planting initiative would be funded out of a broader $3-billion climate fund. A Liberal document said the costs would be offset by the revenues from the Trans Mountain pipeline expansion project, which the government purchased for $4.5-billion.

Shane Moffatt, head of the nature and food campaign at Greenpeace Canada, said the Liberal’s tree-planting proposal is a step in the right direction, but it cannot be a substitute for continuing to take oil out of the ground.

“It’s not a consistent approach to simultaneously be planting trees and continuing with oil extraction at the level we’re seeing now," Mr. Moffatt said.

Mr. Singh also made a climate-related announcement, telling reporters in Ladysmith, B.C., that the NDP would create a $40-million coastal-protection fund if it forms government. He said that the fund would protect salmon, improve Coast Guard equipment and training, and clean up abandoned vessels that pose a threat to Canada’s coastlines. He also reiterated his party’s plan to fight the Liberal government’s purchase of the Trans Mountain project.

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“Instead of Mr. Trudeau, who likes to say pretty words before an election then buy a pipeline, or Mr. Scheer, who doesn’t believe that there even is a crisis, New Democrats are committed … to putting massive investments to fighting the climate crisis and protecting our coastline,” Mr. Singh said.

With a report from Michelle Zilio