Animal lover Debbie Young chained herself to railings surrounding a cow decorated with images of Andy Warhol.
It is the work of Banksy, a celebrated graffiti artist whose exhibition, Turf War, opened yesterday in Hackney, east London.
Celebrity chef Jamie Oliver and DJ Sara Cox are among the artists' fans, and both attended the star-studded launch party.
But Miss Young says painting and exhibiting the animals including a bull and two pigs is cruel.
This morning she entered the gallery space in Kingsland Road and chained her head to the railings with a bicycle lock.
She and two male protesters apparently want to liberate the livestock and take them to an animal sanctuary.
A spokeswoman for the exhibition said the RSPCA and Hackney Council had approved using the animals.
Other exhibits include vandalised monuments and burnt-out squad cars.
Banksy's real name is unknown, but his canvasses can fetch more than £10,000. He is known for a series of stunts such as breaking into the elephant enclosure at London Zoo and spraying the words: "I want out. This place is too cold. Keeper smells. Boring, boring, boring."