When asked about US President Donald Trump and others who have mocked her, the 16-year-old activist said they likely feel their world view and interests are being threatened by climate activism.

"We've become too loud for people to handle so people want to silence us," she said at a rally in Montreal after meeting Canadian Prime Minister Justin Trudeau. "We should also take that as a compliment."

The youth climate movement has drawn criticism from some who accuse the students of overreacting and say they would be better off going to school. In an apparent sarcastic jibe at Thunberg this week following her haranguing of world leaders, Trump tweeted: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"

Instead of addressing Trump by name, she said Friday that she didn't "understand why grown-ups would choose to mock children and teenagers for just communicating and acting on the science when they could do something good instead."

Thousands later chanted "Greta! Greta!" as she spoke at an afternoon rally in Montreal.

"We will do everything in our power to stop this crisis from getting worse even if that means skipping school or work," she said. "The people have spoken. And we will continue to speak until our leaders listen and act. We are the change and change is coming."

Her comments came as students in Italy symbolically torched a replica of planet Earth, one of many protests as part of the climate strikes sparked by the Swedish teen. Some participants echoed the anger she expressed this week at a UN summit in New York.

"How dare you!" read one banner at a rally in Italy's financial hub of Milan, where tens of thousands took to the streets and later gathered around a giant globe to watch it go up in flames.

More than 100,000 people also rallied in Rome, where protesters held up signs with slogans such as "Change the system, not the climate" or just the word "Future."

Fears about the impact of global warming on younger generation s drew fresh protests in India, Spain, Portugal, Sweden, Finland, the Netherlands and Bolivia a week after hundreds of thousands rallied worldwide ahead of the UN summit.

In New Zealand, students marched on Parliament in Wellington, staging one of the largest protests ever held in that capital.

In Canada, Thunberg met Trudeau, who praised her activism on climate change.

"She is the voice of a generation, of young people who are calling on their leaders to do more and do better," Trudeau said. "And I am listening."

Trudeau, who is in the middle of an election campaign, announced a plan to plant 2 billion trees over the next decade.

Thunberg, however, indicated that she expects more, even of leaders who welcome the movement. Scientists this week issued new dire warnings about the consequences of rising temperatures on the world's oceans and cold regions.

Thunberg told a crowd in Montreal it was moving to see people of all generations so passionate for a cause.

"He (Trudeau) is of course obviously not doing enough, but this is just a huge problem, this is a system that is wrong," she said. "My message to all the politicians is the same: Just listen and act on the science."

Australian Associated Press