Thunberg, 16, spearheaded the march on Friday in which Canadian Prime Minister Justin Trudeau also participated along with his family, reports Efe news.
Trudeau met Thunberg privately before the march. After the meeting, the activist said that she told Trudeau he was not doing enough to respond to the climate crisis, though she clarified that it was the same message she conveys to all political leaders.
"My message to all the politic around the world is the same. Just listen and act on the current best available science," said the young activist, who has become the voice and face of the global movement against climate inaction.
Thunberg, who took part in the Montreal march accompanied by Canadian indigenous leaders, addressed the protesters: "At least 500,000 (are) here today. You should be very proud of yourselves (...) because this, we have done together and I cannot thank you enough for being here."
The teenager also came down hard on Canada and her home country, Sweden.
"You are a nation that is allegedly a climate leader, and Sweden is also a nation that is allegedly a climate leader," she said. "In both cases, it means absolutely nothing because in both cases, it's just empty words."
Meanwhile, Trudeau on Friday defended the approval of the controversial Trans Mountain pipeline expansion project and said his government had a "national climate plan that will reduce our emissions and hit our 2030 targets in a way that also includes getting a better price for our oil resources".
Trudeau, which is in the middle of campaigning for the general election scheduled for October 21, also took advantage of the Montreal march to announce that, if re-elected as prime minister, his government will plant 2 billion trees to help fight the climate crisis.
Tens of thousands of people, many of them school students, also took part in climate change protests in other major Canadian cities, including Toronto, Vancouver and Quebec City.
( With inputs from IANS )