Two climate activists on Saturday each glued a hand to the frame of paintings by Spanish master Francisco Goya in Madrid to protest inaction in the face of global warming.
The protest at the famed Prado museum damaged neither painting, but the protesters scrawled "+1,5°C" on the wall between the two artworks and both were detained, police said.
The United Nations warned last week that the world was nowhere near the Paris Agreement target of capping warming at 1.5 degrees Celsius above pre-industrial levels.
Saturday's stunt in Madrid was the latest increasingly daring action taken by climate activists to grab the headlines, including throwing soup on Vincent van Gogh paintings in London and Rome, and mashed potatoes on a Claude Monet masterpiece.
On Sunday, nearly 200 nations will kick off in Egypt's Sharm el-Sheikh the latest climate summit tasked with taming the terrifying juggernaut of global warming.
Climate activist group Extinction Rebellion posted a video online showing the two activists each with a hand fixed on a painting before museum security moved in.
The group said the two artworks in question were "The Naked Maja" and "The Clothed Maja".
The action was to protest rising world temperatures which will "provoke an unstable climate with serious consequences for all the planet", the group said.
Videos posted by Extinction Rebellion show the two young women pulling glue from their clothes and sticking their hands to the frames before addressing other museum goers.
Some of the crowd shout at the activists before security appears and asks those present to stop filming.
- 'Desperate cry' -
Spanish Culture Minister Miquel Iceta denounced the attack, writing on Twitter that it was an "act of vandalism" and that "no cause justifies attacking everyone's heritage".
It is the latest in series of protests by climate activists targeting famous artworks in European cities.
On Friday, a group splashed pea soup onto a 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 those paintings were covered by glass and were undamaged.