More than 150 supporters turned out to greet Greta Thunberg as she came ashore in Lisbon on Tuesday.
Her arrival coincides with the release of a bleak report by the Geneva-based World Meteorological Organization, which warns that the past decade is almost certain to be the hottest yet recorded.
Thunberg missed last Friday's climate strike as heavy winds delayed her arrival in Portugal but she is due to join thousands of activists marching in Madrid on Friday afternoon on the fringes of the COP25.
The conference kicked off on Monday with U.N. Secretary-General Antonio Guterres warned that the planet had reached a "point of no return."
Top priorities include establishing a common time frame for countries to implement their national climate commitment plans, and resolving the issue of international carbon markets. That issue is the only one from the Paris rule book endorsed in 2015 which delegates failed to agree at last year's COP24 in Poland.