Two climate activists on Saturday each glued a hand to the frames of two paintings by Spanish master Francisco Goya at the Prado museum in Madrid, police said.
The protest did not damage either painting, but the protesters scrawled "+1,5C" on the wall between the two artworks in reference to the Paris Agreement target of capping warming at 1.5 degrees Celsius above pre-industrial levels.
Both activists were detained, police said.
Climate activist group Extinction Rebellion posted a video online showing the two activists each with a hand fixed on a painting before the museum's security officials move in.
The group said the two artworks in question were "The Naked Maja" and "The Clothed Maja".
The action was a protest in the face of rising world temperatures which will "provoke an unstable climate with serious consequences for all the planet", the group said in a statement in Spanish.
It is the latest in a number of similar protests by climate activists targeting famous artworks in European cities.
On Friday, a group splashed pea soup onto a Vincent van Gogh masterpiece in Rome.
"The Sower", an 1888 painting by the Dutch artist depicting a farmer sowing his land under a dominating sun, was exhibited behind glass and undamaged.
Four activists were arrested, according to news reports.
The climate activists from Last Generation called their protest "a desperate and scientifically grounded cry that cannot be understood as mere vandalism".
They warned the protest would continue until more attention was paid to climate change.
Other actions have seen cake or mashed potatoes used in recent weeks.
They have targeted masterpieces such as the "Mona Lisa" by Leonardo da Vinci in the Louvre in Paris or "Girl with a Pearl Earring" by Johannes Vermeer at The Hague's Mauritshuis museum.
In October, the group Just Stop Oil threw tomato soup over Van Gogh's "Sunflowers" at London's National Gallery.
All of those paintings were covered by glass and were undamaged.