"You have stolen my dreams and my childhood with your empty words," she told a UN climate summit in New York.
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.
In an emotional speech, 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."
Mr Guterres, who organised the meeting, said the world was "in a deep climate hole" and that urgent action was needed.
"Time is running out, but it's not too late," he said.
German Chancellor Angela Merkel said her country would double to €4bn (£3.5bn; $4.4bn) it financial commitment to fight global warming.
French President Emmanuel Macron said international organisations had pledged to release $500m in additional aid to protect tropical forests.
New Zealand's Prime Minister Jacinda Ardern said things "are starting to turn around" in the country.
"Our gross emissions peaked in 2006, over 80% of our electricity already comes from renewable hydro and wind, and we have begun an ambitious agenda.
"We have introduced in parliament the zero carbon bill, the purpose of which is to ensure New Zealand lives within the threshold of 1.5C of global warming necessary to avoid catastrophic weather events for our Pacific neighbours."