The protesters scaled the roof of the main entrance and unveiled a banner, which read: "18% of greenhouse gases are produced by animal farming. Fight climate change - go vegan."
The activists are from the group Climate Camp, which is also involved at the protests outside the construction of a new coal-fired power station in Kingsnorth, Kent.
Activist Phil Murray, 22, said: "The meat industry is up there with short-haul flights as one of the most environmentally irresponsible sectors."
The group claims that if a person went vegan, they could reduce their carbon footprint by 24,000 calories a day of fossil fuel.
Clare Whitney, another protester, said: "An animal-based diet is no longer sustainable with climate change threatening the planet, its ecosystems and the people who depend on them."
The protests on Monday also coincided with demos outside American air base RAF Mildenhall in Suffolk and outside the offices of mining company BHP Billiton in London.