Swedish youth activist Greta Thunberg had a strong message for world leaders at the UN's climate change summit on Monday, telling them they had stolen her childhood with "empty words".

"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 activist tearfully told world leaders in the audience.

She repeated the message, admonishing them for allowing ecosystems to collapse and saying that they were only concerned about the "money" and "fairy tales of eternal economic growth".

"How dare you continue to look away and come here saying that you're doing enough when the politics and solutions needed are still nowhere in sight?" she said.

Her message came just days after millions of activists marched across the world as part of a protest movement the young activist started outside the Swedish parliament in August 2018.

"You are failing us. But young people are starting to understand your betrayal. The eyes of all future generations are upon you. And if you choose to fail us, I say, we will never forgive you. We will not let you get away with this. Right here, right now, is where we draw the line. The world is waking up, and change is coming whether you like it or not," she added.

Her speech quickly went viral, along with a video of Thunberg reacting to US President Donald Trump walking past her.

After delivering an impassioned speech at the summit, Thunberg joined 15 other children at UNICEF headquarters to announce that they had filed a complaint with the UN Committee on the Rights of the Child alleging that five countries violated their rights by not doing enough to address the climate crisis.

“World leaders are failing to protect the rights of the child by continuing to ignore the climate and ecological crisis,” Thunberg said at UNICEF headquarters.