Greta Thunberg has marched alongside an estimated 500,000 people at a global climate strike demonstration in Montreal, telling protesters she did not understand why adults mock children for campaigning against global warming.

Donald Trump had taken aim at the teenager earlier this week, tweeting sarcastically: “She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!”

Mr Trump's favourite TV network, Fox News, was forced to apologise to the teenager after one of its guests suggested she was mentally unwell. And on Friday Bernard Arnault, the richest man in Europe, claimed the 16-year-old's efforts were "demoralising" for young people.

Asked about the criticism, Greta said: “We’ve become too loud for people to handle so people want to silence us. We should also take that as a compliment.”

The Swedish climate activist did not address Mr Trump by name but told protesters she did not ”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”.

Greta Thunberg inspires climate activists everywhere: In pictures Show all 12 1 /12 Greta Thunberg inspires climate activists everywhere: In pictures Greta Thunberg inspires climate activists everywhere: In pictures In the protest that started a movement, Greta skips school to sit outside of the Swedish parliament in Stockholm in order to raise awareness of climate change on 28 August 2018 Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta speaks at the World Economic Forum in Davos on 25 January AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta stages a protest at the World Economic Forum in Davos on 25 January Reuters Greta Thunberg inspires climate activists everywhere: In pictures Greta speaks at the House of Commons in London on 23 April PA Greta Thunberg inspires climate activists everywhere: In pictures Greta addresses to the occupation at Marble Arch in London on 21 April AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta meets the pope on a visit to Rome Reuters Greta Thunberg inspires climate activists everywhere: In pictures Greta speaks at the senate in Rome on 18 April Reuters Greta Thunberg inspires climate activists everywhere: In pictures Greta addresses a debate of the EU Environment, Public Health and Food Safety committee at the European Parliament in Strasbourg on 16 April AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta receives the Special Climate Protection Award at the German Film and Television awards in Berlin on 30 March AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta attends a children's climate protest in Berlin on 29 March AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta addresses a children's climate protest on 1 March in Hamburg Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta attends a meeting for the Civil Society For rEUnaissance at the EU Charlemagne Building in Brussels on 21 February AFP/Getty

“We’re at least 500,000 here today – you should be very proud of yourself,” the teenager told the Montreal crowds, to cheers.

“We have done this together and I can’t thank you enough for being here. It’s just incredible to be united in such a way for a common cause. It just feels great, doesn’t it?”

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Greta was joined in Montreal by aboriginal Canadians, a group she described as “often the ones who are at the front line” of global warming.

She also met with Justin Trudeau, the Canadian leader, privately for around 15 minutes. “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.”

Greta began her activism a year ago, by protesting alone outside the Swedish parliament. Her campaign has since galvanised thousands of schoolchildren to take part in protests demanding action to save the environment.

Students conducted a second wave of global protests on Friday, with tens of thousands taking to the streets in New Zealand.