Thunberg, young climate activists blast inaction at the U.N.

"The world signed a contract between generations that the present world would leave a world worth inheriting to the future. And today, I want to tell the world you, you are defaulting on that contract and we're here to collect," said U.S. teen climate activist Alexandria Villasenor.

"They promised to protect our rights and they have not done that," said Thunberg.

Days after millions of young people took to the streets worldwide to demand emergency action on climate change, leaders gathered at the United Nations on Monday to try to inject fresh momentum into stalling efforts to curb carbon emissions.