In an emotional speech at the UN climate summit in New York, she said: "This is all wrong. I shouldn't be up here. I should be back in school on the other side of the ocean, yet you all come to us young people for hope.
How dare you?
"You have stolen my dreams and my childhood with your empty words," the 16-year-old said.
And she urged world leaders to act urgently, saying: "We will be watching you."
About 60 world leaders are taking part in the one-day meeting organised by UN Secretary General António Guterres.
He earlier said countries could only speak at the summit if they came with action plans to cut carbon emissions.
US President Donald Trump, a climate change sceptic, had not been expected at the meeting - but he was briefly spotted in the audience.
Brazil and Saudi Arabia are among the countries staying away.