Fresh off the climate strike that took hundreds of thousands of young people out of classrooms and into the streets globally, youth leaders gathered at the United Nations to demand radical moves to fight climate change.

"We showed that we are united and that we, young people, are unstoppable," said Swedish 16-year-old activist Greta Thunberg, who started the climate strike movement. During Thunberg's short lifetime, the Earth has already warmed 0.34C.

More than 700 mostly young activists attended the first of its kind Youth Climate Summit, according to Luis Alfonso de Alba, the UN special climate summit envoy.

Swedish environmental activist Greta Thunberg walks around UN building after attending the Youth Climate Summit. Photo / AP
Swedish environmental activist Greta Thunberg walks around UN building after attending the Youth Climate Summit. Photo / AP

The strike across six continents and the youth conference presage a full-on climate conference this week at the UN General Assembly, which has placed the issue of climate change at front and centre as world leaders gather for the annual meeting.

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"Stop the criminal contaminant behaviour of big corporations," said Argentine climate activist Bruno Rodriguez. "Enough is enough. We don't want fossil fuels anymore."

Jayathma Wickramanayake, the UN Secretary-General's youth envoy, called climate change "the defining issue of our time. Millions of young people all over the world are already being affected by it."

Climate activist Greta Thunberg tripped up a Republican congressman who tried to catch her with a flawed analogy at yesterday's climate change hearing. Video / C-Span

Fiji climate activist Komal Karishma Kumar said: "Young people from different parts of the world are living in constant fear and climate anxiety, fearing the future, the uncertainty of a healthy life or a life for their children at all. I do not want our future generations to submerge with our sinking islands."

- AP