Climate activist Greta Thunberg says she feels "energized" after spending three weeks crossing the Atlantic in a catamaran.
Some 20 activists cut off traffic in central Madrid and staged a brief theatrical performance to protest climate change.
Members of the worldwide group called Extinction Rebellion held up a banner in Russian that read: "Climate Crisis. To take action immediately". I have been on quite an adventure, of course I, as well as the other climate activists, we will not stop, we will continue to do whatever we can, continue to travel around, and to put pressure on people in power and to make sure that they prioritise this the highest. "What do we want?"
Greta Thunberg talks to reporters after her arrival to NY in August.
Greta missed last Friday's climate strikes as heavy winds delayed her arrival to Lisbon, but she is due to join thousands of activists marching in Madrid this week on the fringes of the COP25.
Her arrival in Lisbon coincides with the release of a bleak report by the Geneva-based World Meteorological Organization, which warns that the past decade is nearly certain to be the hottest yet recorded.
Thunberg hitched a renewable-energy ride from the United States, joining an Australian family on their 48-foot (15-metre) yacht.
Thunberg, her father, and the crew aboard La Vagabonde sailed for 20 days from NY on a yacht that saved on carbon emissions and reduced their carbon footprint.
In a report released on the sidelines of this year's United Nations climate change conference in Madrid, the agency said this continues the trend that "since the 1980s, each successive decade has been warmer than the last".
Levels of greenhouse gases including carbon dioxide hit record levels previous year, the World Meteorological Organisation has said.
While full-year figures are not released until March, 2019 is also expected to be the second or third warmest year since measurements began, with 2016 still holding the record, it said.