In Europe, students packed streets and squares in London, Copenhagen, Rome, Vienna, Zurich and Lisbon.
The group is also chanting: "You should be embarrassed that we pulled out of of Paris", a reference to President Trump's decision to withdraw the United States from the Paris Climate Accord.
The Friday protest comes ahead of tomorrow's planned "March of the Century", another similar climate change protest coordinated to take place on the same day as the gilets jaunes Acte 18 movement in Paris and beyond.
But, like many other speakers, Ider also was hopeful that her generation is passionate about these issues and seemed inspired by the group present today. They were there to rally and to help each other.
Ms Thunberg said: "We have only been born into this world, we are going to have to live with this crisis our whole lives. So will our children and grandchildren and coming generations", Thunberg said.
Lana, 13, said: "They're messing up our future and we're the ones who are going to have to clean it up, so I think it's important that we come and tell them about it".
"The main thing we want people to realise is that we are not only asking the Hong Kong government to do more, it is also people themselves who need to do more", said Zara Campion, 17, co-organiser of the strike.
He says critics of the last march - some of whom labelled pupils missing lessons to take part as "truants" - has strengthened his resolve to campaign for a greener future.
A new report from global experts is ringing the alarms of climate change. The measures include a law to require action that would limit global warming to 1.5 degrees Celsius, a shift away from fossil fuels and investment in post-carbon renewable energy.
More than 220 rallies were held in Germany. In Duesseldorf, some 2,000 schoolchildren paraded with a carnival float depicting a giant effigy of Thunberg with the words "Do something about the climate catastrophe at last", written along her raised arms.
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But rather than taking these more nuanced tacks, she is, for all intents and purposes, taking impeachment off the table. Al Green , the Texas Democrat who introduced impeachment articles a year ago , said he will try to force a vote.
Some students kicked off the demonstration by walking out of class at 11:11 a.m. Friday, a reference to the 11 years it could take for global warming to reach 1.5 degrees Celsius above pre-industrial levels, as projected by the Intergovernmental Panel on Climate Change. "Real change starts small and builds on itself a little at a time", she said.
Thousands of young people marched through Brighton chanting loudly and holding a variety of banners.
Some politicians praised the students.
Students hold signs protesting pipelines and paying tribute to victims of natural disasters. Thailand is one of the world's worst marine plastic polluters.
The global Youth for Climate movement has been gaining traction in France in recent weeks, after notice of today's major protest was first called for on February 14.
Some climate specialists say while they are encouraged by the actions and enthusiasm of the students, but that climate change is far more complex than what they're being told.
In contrast, officials of neighboring New Zealand endorsed the protesters, with the country's PM Jacinda Ardern stating that the youth should not wait for voting age to be heard.
Some politicians have criticized the students, suggesting they should be spending their time in school, not on the streets.
In Australia, Education Minister Dan Tehan said the strike was "not something that we should encourage".