Greta Thunberg 'energized' after completing Atlantic voyage to Lisbon
Thunberg, who refuses to fly to avoid harming the environment, arrived on Tuesday onboard a catamaran that left the United States last month.
"It feels good to be back," Thunberg said, adding she felt "energized."
Lisbon mayor Fernando Medina said it was a privilege to welcome Thunberg to the city.
A large crowd of young people also welcomed her to Lisbon, where she carried her iconic hand-painted sign declaring a school strike.
Thunberg is planning to continue onwards to Madrid where the UN climate conference, COP25, opened on Monday and is set to run until December 13.
"We will continue the fight there, so that the voices of the people are being heard there," Thunberg added.
On Friday, a new wave of global climate protests are planned, including in central Madrid. It is likely Thunberg will attend.
Thunberg in August 2018 began a "school strike" outside the Swedish parliament that inspired a youth-led movement, Fridays for Future, which has staged climate strikes across the globe.
This has been Thunberg's second boat trip across the Atlantic within four months. She caught a lift over on a racing yacht captained by German sailor Boris Herrmann and Pierre Casiraghi of Monaco, in a zero-carbon journey that took 15 days.
Her return voyage was on the 15-metre-long La Vagabonde with an Australian "influencer" couple with 1.2 million YouTube subscribers.
In addition to Australians Riley Whitelum and Elayna Carausu, their 11-month-old son, Thunberg's father Svante, and English skipper Nikki Henderson were on the catamaran.
Whitelum said the voyage had been challenging at times with high waves.
Thunberg said she had been seasick on the first day, but had recovered and had adjusted to being on board the catamaran.
"You get very relaxed in a way," she said of the three-week voyage.
