MONTREAL • Teen activist Greta Thunberg has hit back at critics - including US President Donald Trump - saying their mockery of children shows that her message has become "too loud to handle".

The 16-year-old Swede also told a huge Montreal rally on Friday that world leaders had disappointed young people with empty words and inadequate plans.

"Today we are millions around the world, striking and marching again, and we will keep on doing it until they listen," Greta told a crowd that organisers estimated to be half a million strong. Officials put the total number at closer to 315,000, the BBC said.

Mr Trump had mocked Greta last week, and Canadian MP Maxime Bernier had 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," Greta said before the rally. "We should take that as a compliment."

On Friday, the climate strikes that she inspired started in Asia and continued in Europe. Tens of thousands of students kicked things off in New Zealand.

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

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

Meanwhile, Coordinating Minister for Infrastructure and Minister for Transport Khaw Boon Wan, who was in Montreal, wrote on his Facebook page that with a meeting of the International Civil Aviation Organisation suspended, he headed out to the rally.

REUTERS