In August 2018, Greta Thunberg sat outside Swedish parliament with a sign reading “school strike for climate.” It was her first Friday walking out of school for climate action, and she made a weekly routine out of it.

Today her efforts inspired millions to march in the streets of cities across the world. But those first Fridays were lonely affairs. She stationed herself outside Swedish parliament with her handwritten sign and, sometimes, a few friends.

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Foto: Fifteen year old Swedish student Greta Thunberg leads a school strike and sits outside of Riksdagen, the Swedish parliament building, in order to raises awareness for climate change on August 28, 2018 in Stockholm, Sweden.sourceMICHAEL CAMPANELLA/Getty Images

Thunberg will speak on Saturday at the United Nations Youth Climate Summit in New York City, then at the UN Climate Action Summit on Monday.

On Friday, she joined the protesters she'd inspired in New York City.

"I would never have predicted or believed that this was going to happen someday. And so fast, only in 15 months," Thunberg told the AP. "I can't wait to see the official numbers come in. It will be magnificent."

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Foto: Thunberg will make her voice heard again on Saturday at the United Nations Youth Climate Summit in New York City, then speak at the UN Climate Action Summit on Monday.sourceREUTERS/Shannon Stapleton

New York City mayor Bill de Blasio estimated 60,000 people marching through the streets of Lower Manhattan. Other estimates were as high as 250,000 people.

"We have only been born into this world, we are going to have to live with this crisis our whole lives. So will our children and grandchildren and coming generations," Thunberg told Reuters. "We are not going to accept this. We are striking because we want a future and we are going to carry on."

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Foto: sourceDavid Slotnick/Business Insider

"I think if enough people get together and stand up for this then that can have a huge difference, to put pressure on the people in power, to actually hold them accountable and to say you need to do something now," Thunberg said.