"Unless we take positive action, the future's looking bleak for those of us that have grown up in an era defined by climate change".
One 15-year-old Oxfordshire student, blogger and author, Alex White, tweeted: "I've been upfront with school and told them I will be walking out tomorrow lunch".
A group of teens holding a placard reading "We're only here for a day off skl", said: "We think that climate change is bulls**t - there's too many feminazis here, too".
Anna Taylor of the UK Student Climate Network, which helped coordinate the protests, said: "We're running out of time for meaningful change and that's why we're seeing young people around the world rising up to hold their governments to account on their dismal climate records".
While the Liberal Democrat education spokeswoman, Layla Moran, a former teacher, joined a protest in Oxford, the shadow education secretary, Angela Rayner, was more nuanced in her support: "I'm inspired by the many young people who want to support the #schoolstrike4climate movement, but I hope it can evolve so we can build on its success without the loss of time in the classroom".
Ms Lucas addressed campaigners in Brighton, saying they should be allowed to miss school because of "exceptional circumstances". "We have come here to let them know that change is coming whether they like it or not".
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However, at least one person in attendance was alarmed enough to report Gonzalez to internal investigators. He wrote on the micro-blogging platform: "This guy Emanuel Gonzales is a dirty cop abusing his POWER ".
Labour leader Jeremy Corbyn, saw it differently, saying it was "inspiring to see them making their voice heard".
Organisers Youth Strike 4 Climate revealed there were an estimated 15,000 people taking part in the protests across the United Kingdom on Friday.
"I do want to slightly caution that with the more official view that we can't put any more burdens on our superb teachers and teaching staff".
This is week 26 of her climate strike.
But a string of major climate reports have cast doubt over the entire process to avert runaway global warming, suggesting the Paris goals fall well short of what is needed.
Mr Greer also criticised the Scottish Government's Climate Bill, which seeks to make Scotland carbon neutral by 2050, as "not almost strong enough". "We have to take it upon ourselves to make a difference", she added.
London mayor Sadiq Khan, speaking at the National Clean Air Summit at the Tate Modern on Thursday, said he understands why children feel "very strongly" about pollution and climate change and called for politicians to take "great strides to clear up the air across our country". Every school student who takes action against the climate crisis on Friday is doing exactly that. "If no one else is going to, we have to do it", said Kemy, 16, a student from Woodhouse College in north London.
Scotland Yard said two arrests were made in London.