Videos posted on social media appear to show a young man in a wig and lipstick who had arrived in a wheelchair at the Louvre Museum in Paris.
He shouted at people to think of planet Earth before targeting the famous artwork. The man, whose identity was unknown, was also seen throwing roses in the museum gallery.
The cake attack left a conspicuous white creamy smear on the glass but the famous work by Leonardo da Vinci was unharmed.
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Security guards were filmed escorting the wig-wearing activist away as he called out to the surprised visitors in the gallery: “Think of the Earth. There are people who are destroying the Earth. Think about it. Artists tell you: think of the Earth. That’s why I did this.”
Guards were then filmed cleaning the smeared cream from the glass. Officials at the Louvre were not immediately available for comment.
The 16th century Renaissance masterpiece has been targeted before.
The painting was stolen in 1911 by a museum employee, an event which increased the painting’s international fame.
It was also damaged in an acid attack perpetrated by a vandal in the 1950s, and has since been kept behind glass.
In 2009, a Russian woman who was angry at not being able to get French citizenship threw a ceramic cup at it, smashing the cup but not harming the glass or the painting.