Two Goya paintings in Spain are the latest to be targeted by climate campaigners in a protest that has been repeated in museums across Europe.
A man and a woman attached themselves to Spanish master Francisco de Goya's The Clothed Maja and his The Naked Maja, and painted "+1.5 C” on the wall between the two works in Madrid's Prado museum.
Similar protests have used masterpieces in London, Paris and Amsterdam with a number of groups claiming responsibility.
Campaign group Futuro Vegetal said its members carried out the protest.
“Last week, the UN recognised the impossibility of keeping us below the limit of 1.5 º Celsius (set in the 2016 Paris climate agreement). We need change now,” it wrote on Twitter.
The Prado said its paintings had not been damaged, and the graffiti on the wall was quickly painted over.
“We condemn the use of the museum as a place to make a political protest of any kind,” the gallery added.
Police said two people had been arrested.
Videos posted by the Extinction Rebellion group show two 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.
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 campaigning targeting famous artworks in European cities.
On Friday, a group splashed pea soup on to 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 those paintings were covered by glass and were undamaged.