Greta Thunberg arrived in Lisbon, Portugal, on Tuesday after crossing the Atlantic for the second time this year.
"People are underestimating the force of angry kids," Thunberg told a crowd of reporters and supporters after she disembarked. "They're angry and frustrated."
The 16-year-old activist spoke after arriving at a port in Lisbon, after more than two weeks spent crossing the Atlantic in a catamaran.
Thunberg said she would spend a few days in Lisbon before making her way to Madrid, where the COP25 climate summit is currently underway, where she would work to ensure the "voices of future generations" are heard.
Thunberg sailed on a 48ft catamaran called La Vagabonde, with an Australian couple who are travelling the world with their newborn child by boat and posting about their experience on YouTube.
She had been in the US since August, where she met with climate activist groups across North America. Thunberg arrived in New York on another sailboat which had given her a carbon-neutral lift from Europe.
The young activist had originally planned to stay in America much longer, slowly making her way to South America, where Chile was expected to host the COP25 meeting.
But after a wave of protests erupted in Chile the country announced that Cit could no longer host the talks, and Spain offered Madrid as an alternative venue.
This left Thunberg in need of a boat ride back to Europe.
"It turns out I've travelled half around the world, the wrong way", she tweeted, adding: "If anyone could help me find transport I would be so grateful."