Swedish teen activist Greta Thunberg—fresh off a two-week transatlantic journey on a fossil fuel-free vessel—joined other climate protesters in New York City on Friday to continue the demand for bold action to address the planetary crisis.

The dozens of activists gathered outside the United Nations headquarters held placards saying "Act now" and "Our future," and chanted messages including "System change, not climate change."

ABC News, on Twitter, shared live video of Thunberg and other youths taking part in the protest, who urged, "Don't just watch us; join us."

The Guardian posted a livestream of the protest:

Among the climate activists in the crowd was 14-year-old Alexandria Villaseñor,  who's been outside the U.N. headquarters every Friday for more than nine months as part of the school strike for climate movement catalyzed by Thunberg.

Thunberg arrived in New York Wednesday on the Malizia II with her father, sailors Pierre Casiraghi and Boris Herrmann, and a documentary filmmaker. Her time in the Americas is set to include taking part in the global climate strikes begining Sept. 20 and participating in the COP25 climate conference in Santiago, Chile, in December.

"The climate and ecological crisis is a global crisis, and the biggest crisis humanity has ever faced," Thunberg told a crowd of supporters upon her arrival. "And if we don't manage to work together to cooperate... despite our differences, then we will fail. So we need to stand together."

"Let's not wait any longer," she said. "Let's do it now."