Climate activist Greta Thunberg said that adults should stop making young people "angry" over global warming.
Ms Thunberg was speaking after her arrival in Lisbon, Portugal, after a two-weeks-plus journey across the Atlantic from her starting point in Virginia, US.
"People are underestimating the force of angry kids," she told reporters.
The 16-year-old is on her way to the COP25 climate summit in Madrid.
She is taking a stand on more polluting forms of transport by sailing, rather than flying or travelling in cars.
Responding to a question from a journalist who said some adults viewed her as "angry", Ms Thunberg said: "We are angry, we are frustrated and it's because of good reasons.
"If they want us to stop being angry, maybe they should stop making us angry."
She had originally planned to travel from the US to a UN climate summit in Chile.
But the South American nation had to give up the event due to civil unrest.
The venue changed to Spain, and so Ms Thunberg hitched a ride on a 48ft sailing catamaran called La Vagabonde.
She travelled with Australian YouTubers Riley Whitlum and Elayna Carausu, as well as Briton Nikki Henderson - who is a professional yachtswoman.
Their boat uses solar panels and hydro-generators for power. However the emissions impact of the voyage has been called into question by reports that suggested Ms Henderson flew to the US from Britain to undertake the trip.
Meanwhile, in a report released on Tuesday during COP25, the World Health Organization (WHO) called on countries to prioritise funding to deal with the effects of climate change on human health. In coming decades, global warming is expected to cause thousands of additional deaths each year from malnutrition, insect-borne disease and heat stress.
WHO researchers surveyed 101 nations to find out which had already developed health and climate change strategies, and whether these plans had sufficient financial backing.
It found about half of the surveyed countries had drawn up a national strategy. But of 45 countries subjected to more detailed analysis, less than 40% said their current health budget fully or partially covered the estimated costs of implementing their national plans. Only 9% had allocated enough resources to carry out their strategies in full.
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