Climate movement now 'too loud to handle' for Trump and critics, says Greta Thunberg

Teenage activist Greta Thunberg hit back at critics including US President Donald Trump on Friday before she marched in a climate strike in Montreal, saying their mockery of children shows her message has become "too loud to handle".

The 16-year-old Swede spoke shortly before the march began alongside aboriginal Canadians, who she invited to lead the march with her because "they are often the ones who are at the front line" of global warming.

Trump mocked Thunberg this week and Canadian Member of Parliament Maxime Bernier called her alarmist and mentally unstable.

"I guess they must feel like their world view or their interests or whatever... is threatened by us. We've become too loud for people to handle so they try to silence us," she told reporters. "We should also take that as a compliment."

On Friday, the climate strikes she inspired started in Asia and continued in Europe. Tens of thousands of students kicked things off in New Zealand, and Thunberg tweeted images from marches held in dozens of cities around the world.

"The strikes have been very successful today," she said. "It is very moving to see everyone, everyone who is so passionate."

Thunberg - who has inspired millions of young people to take to the streets over climate change - denounced world leaders earlier this week for failing to take effective action on the issue.

Matthew McMillan, 22, a mechanical engineering student at Montreal's Concordia University, held a "Make America Greta Again" poster as he joined other students at the protest.

"I think it is the most important event of our generation," he said.

Holding placards and chanting "protect the planet", nine-year-old Xavier Damien Tremblay joined his three younger siblings and mother Maude Richard in the march. "It's our planet that's at stake," he said.

Watch: Climate activist Greta Thunberg glares at US President at United Nations

Trudeau tells Thunberg Canada must do more

Thunberg's presence in Canada coincides with campaigning ahead of next month's federal election. She met with Prime Minister Justin Trudeau privately for about 15 minutes in his capacity as a leader of the government, not the Liberal Party.

Later she was asked what her message to Trudeau had been.

"He is of course obviously not doing enough but... this is such a huge problem," she said. "My message to all the politicians is the same, to just listen to the science, act on the science."

Trudeau is promising more climate action but has also had to defend his decision to buy and expand a major oil pipeline in western Canada.

"You are the problem! Climate criminal!" shouted a man who police hauled away from one of Trudeau's campaign stops. As the prime minister joined the Montreal march, police tackled another man who lunged at him to throw eggs.

Video showed Trudeau consoling his son, who appeared to be frightened and was crying after the incident.

Of his meeting with Thunberg, Trudeau said he had a "wonderful conversation with Greta" and that they "talked directly about the need to do more, much more".

Aviation leaders meet in Montreal

United Nations aviation leaders are in Montreal on Friday attending a meeting of the International Civil Aviation Organisation (ICAO), which is debating ways to minimize the sector's impact on climate.

Commercial flying accounts for 2.5% of carbon emissions, but passenger numbers are forecast to double by 2037, so experts say emissions will rise if more is not done.

ICAO expressed "enthusiasm and support" for the march on Thursday, adding that "more action and faster innovation are now required to address aviation's near- and long-term impacts".