Swedish climate activist Greta Thunberg speaks to AFP during an interview aboard La Vagabonde, the boat she will be taking to return to Europe, in Hampton, Virginia. (AFP)

Swedish teenage eco-warrior Greta Thunberg arrived in Lisbon on Tuesday after crossing the Atlantic in a catamaran to attend a UN climate change summit in Madrid.

The 16-year-old was greeted by dozens of environmentalist activists who waited several hours for her to dock in Portugal’s capital.

She arrived at 12:45 GMT to a welcome ceremony attended by Lisbon mayor Fernando Medina.

Thunberg, 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.

She had expected to be heading for Chile, but the South American nation passed the hosting of the COP25 summit meeting to Spain after suffering a spate of deadly anti-government protests.

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 has not yet decided when or how she will leave Portugal to get to Madrid, a spokesperson said, adding that one option could be an overnight train to the Spanish capital.

The climate summit, which opened Monday and runs to December 13, brings together representatives of some 200 signatory nations to the 2015 Paris accord.

Thunberg rose to prominence after she started spending her Fridays outside Sweden’s parliament in August 2018, holding a sign reading “school strike for climate”.

Having used 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 home.

She is expected to join a climate march in Madrid on Friday.