Hundreds of thousands of people thronged the streets of Montreal on Friday in a climate march that turned the city's downtown into a sea of placard-waving protesters.

To deafening cheers, Swedish environmental activist Greta Thunberg addressed the crowd at the end of the protest.

"We are not in school today, you are not at work today, because this is an emergency, and we will not be bystanders," said the 16-year-old, whose activism has made headlines around the world.

"Some would say we are wasting lesson time. We say we are changing the world. So that when we are older we will be able to look our children in the eyes and say that we did everything we could back then."

Organizers said 500,000 people attended the march, making it the largest in the province's history.

Hours after protesters first gathered at the foot of Mount Royal in Montreal's Plateau district, at noon ET, the streets remained jammed with people.

Watch drone footage of the Montreal march:

Watch as hundreds of thousands of people march in Montreal for the climate, as seen from a drone. 1:38

Marches were also held around the world and elsewhere in Canada, including in Vancouver, Toronto, Ottawa and Quebec City.

The event comes in the middle of the federal election campaign in which the environment has emerged as a key issue.

Young activists — many not old enough to vote — demanded politicians adopt a climate action plan to reduce carbon dioxide emissions to zero by 2050.

Earlier Friday, Thunberg met with Indigenous youths ahead of the march, as well as with Liberal Leader Justin Trudeau.

She said afterward it was clear he hadn't done enough as prime minister.

"My message to all the politicians is the same: to just listen to the science and act on the science."

Inspiring students

The driving force behind the march was students, and classes at many high schools, colleges and universities were cancelled for the day.

Thunberg's presence in Montreal created even more interest in the march.

"It's very inspirational of her to come here," said Maya Jonkov, who is the same age as Thunberg, and attended with several friends.

"I think she's really going to show us that, yes, we need to take action about climate change."

Many of the protesters were students. Classes at many schools, colleges and universities were closed for the day. (Ivanoh Demers/Radio-Canada)

Montreal Mayor Valérie Plante, who has been outspoken about climate change and made public transit free for the day, gave Thunberg the keys to the city following the march.

Many Montreal businesses, unions and community organizations also closed so their employees could attend the demonstration.

"Just know: every little thing we do counts, but the big things have to be done too," said Cedric Gray-Lehoux, a member of the First Nations of Quebec and Labrador Youth Network.

"We have to make a drastic change. We have to change the system. We have to say that enough is enough."

Culmination of yearlong series of strikes

Ben Clarkson, a spokesperson for La Planète s'invite au Parlement, said the Montreal gathering is the culmination of a yearlong series of climate strikes, which began in August 2018.

La Planète is one of the groups organizing Friday's climate march whose name translates as "the planet goes to Parliament."

More than 150 countries have participated in the #FridaysForFuture movement, launched by Thunberg.

Clarkson said Montreal's protest movement is particularly powerful, and he's hopeful it will lead, ultimately, to significant change.

"We have a history of popular mobilization that the rest of Canada and maybe the rest of North America doesn't have here in Montreal," he said. "That's been able to turn into and form into a very effective organizational group that has really grabbed onto this one day."

Trudeau, Green Party Leader Elizabeth May and Bloc Québécois Leader Yves-François Blanchet all took part in Friday's march in Montreal.

The NDP's Jagmeet Singh was slated to attend a climate march in Victoria, while Conservative Leader Andrew Scheer did not plan to attend a march while campaigning in Vancouver.