Thunberg in Portugal after ocean crossing

Swedish teen climate activist Greta Thunberg, who refuses to fly to avoid harming the environment, has arrived in Lisbon after sailing across the Atlantic on an Australian couple's yacht.

"It feels unreal. I'm of course very happy to reach land again," she told Swedish daily Dagens Nyheter shortly before landfall on Tuesday.

She is due to continue to Madrid to attend the UN climate conference COP25 that began on Monday. The summit is set to run until December 13.

The 16-year-old had travelled to America to attend the major COP25 climate summit in Chile, among other events, but the conference was moved to Madrid due to unrest in that country.

She hitched a ride with Australian YouTube influencers on the 15-metre-long La Vagabonde from the US state of Virginia on November 13.

Thunberg staged her first "school strike" in 2018 outside the Swedish parliament, inspiring a youth-led movement that has held climate strikes worldwide.

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