Swedish teenage eco-warrior Greta Thunberg sailed into Lisbon on Tuesday after crossing the Atlantic in a catamaran with her sights set on the UN climate change summit in Madrid.

Lisbon mayor Fernando Medina was on hand to welcome her following her 20-day voyage from the United States where she attended the UN climate summit in New York in September.

"We're going to COP25 in Madrid and we'll continue to fight there to make sure that... the voices of the people are being heard and voices of the future generations are being heard," Thunberg said.

IMAGESSwedish teen eco-warrior Greta Thunberg arrives in Lisbon, Portugal, after crossing the Atlantic in a catamaran to attend a UN climate change summit in Madrid. Thunberg, 16, has become the face of young peoples' demands for climate action and made a IMAGESSwedish teen eco-warrior Greta Thunberg arrives in Lisbon, Portugal, after crossing the Atlantic in a catamaran to attend a UN climate change summit in Madrid. Thunberg, 16, has become the face of young peoples' demands for climate action and made a point of making the journey back from September's New York climate summit by sea rather than fuel-guzzling plane. Photo: AFPTV / Jérome PIN

The Swede who has become the face of young people's demands for climate action made a point of returning to Europe by sea rather than fuel-guzzling plane.

She had expected to be heading for Chile, but the South American nation handed over the hosting of the COP25 summit to Spain in the face of deadly anti-government protests.

The teenager said she felt "energised" after the trip and would spend a few days in Lisbon before attending the Madrid summit The teenager said she felt "energised" after the trip and would spend a few days in Lisbon before attending the Madrid summit Photo: AFP / CARLOS COSTA

Thunberg, British navigator Nikki Henderson and the Australian family who own the vessel set sail back to Europe on the 14-metre (45-foot) La Vagabonde catamaran on November 13 from Hampton, Virginia, on a voyage of more than 5,500 kilometres (3,400 miles).

The teenager said she felt "energised" after the trip and would spend a few days in Lisbon before attending the Madrid summit, which runs until December 13.

Dozens of environmental activists greeted Greta as she arrived in Lisbon Dozens of environmental activists greeted Greta as she arrived in Lisbon Photo: AFP / CARLOS COSTA

She will join a climate march there on Friday.

Thunberg has not yet decided when or how she will travel to Madrid, a spokesman said, adding that one option was an overnight train to the Spanish capital.

The COP25, which opened Monday, brings together representatives of some 200 signatory nations to the 2015 Paris accord.

"Many people say that nothing will come out of the COP25... But that (should) never be the reason to not try to do everything you can to make sure this outcome is different," she said.

"People are underestimating the force of angry kids. We are angry and frustrated, but for good reason," she added.

After using her appearance in New York to denounce governments for failing to act with sufficient urgency on the environment, Thunberg attended weekly green protests in several US cities before embarking on her long trip back to Europe.