(CNN) Greta Thunberg doesn't mince words. Not even when addressing the world's most powerful people.

"We are in the beginning of a mass extinction and all you can talk about is money and fairy tales of eternal economic growth," the Swedish climate activist told the United Nations General Assembly on Monday. "How dare you?"

Speaking during the UN Climate Action Summit in New York, the 16-year-old was visibly frustrated with her audience and at times appeared to be holding back tears of anger.

"You have stolen my dreams and my childhood with your empty words and yet, I'm one of the lucky ones," Thunberg told the assembly, her signature long braid swept to the side. "People are suffering, people are dying."

Thunberg's message to the leaders was clear. Like many times in the past , she accused them of not doing enough to mitigate climate change. "For more than 30 years, the science has been crystal clear. How dare you continue to look away?"

For more than 30 years, the science has been crystal clear. How dare you continue to look away?" Greta Thunberg

The Swedish schoolgirl rose to prominence because of her determination to persuade global leaders to take climate change seriously.

She started with weekly sit-ins outside the Swedish Parliament, holding a handmade "School Climate Strike" sign. In just a few months, the one-girl protest grew into a worldwide movement , with students walking out of schools in well over 100 countries.

Environmental activist Greta Thunberg addresses the Climate Action Summit at the United Nations General Assembly.

Thunberg is taking a sabbatical year from school to attend conferences and meetings with policymakers and those impacted by climate change.

But persuading her to come to America wasn't easy. Thunberg refuses to fly because of the high levels of emissions from air travel. When she traveled to the World Economic Forum in Davos, Switzerland, in January, she traveled by train. It took her 32 hours.

"Adults keep saying we owe it to the young people to give them hope. But I don't want your hope. I want you to panic," she told the rich and powerful gathered in the Swiss mountain resort. Like in New York on Monday, her speech was met with a stunned silence, then an overwhelming applause.

To get her to come to New York to address the United Nations, she was offered the option of sailing across the Atlantic Ocean on a 60-foot zero-emissions yacht.

Separately on Monday, Thunberg and 15 other children filed a complaint with the United Nations alleging that five of the world's leading economies have violated their human rights by not taking adequate action to stop the unfolding climate crisis.

The petition names five countries -- Germany, France, Brazil, Argentina and Turkey -- that they say have failed to uphold their obligations under the Convention on the Rights of the Child, a 30-year-old human rights treaty that is the most widely ratified in history.

She said her message to the global leaders gathered in New York is simple: "We are watching you."

"If you chose to fail us, I say we will never forgive you," she added.