More than 600 artworks critiquing corporate sponsors of the UN summit on climate change have been installed in advertising spaces across Paris.
The Brandalism campaign said it was behind the unauthorised artworks.
It said in a statement the aim was to "highlight the links between advertising, consumerism, fossil fuel dependency and climate change".
The action comes as demonstrations take place around the world to demand action to stop climate change.
Prominent corporate sponsors of the talks have been targeted by the posters, which say that they are "part of the problem".
Several others feature world leaders, nearly 150 of whom are due to attend the Paris talks.