Animal rights activists chained themselves to a Victorian pig slaughterhouse 'gas chamber' on Monday morning to protest against carbon dioxide stunning in the pork industry.

Four protesters walked into Diamond Valley Pork Slaughterhouse in Laverton at about 2am, sneaking into the area where carbon dioxide (CO₂) is used to render a pig unconscious just before being slaughtered, and shut down the abattoir for hours before police moved in.

About 50 protesters continued the demonstration outside the slaughterhouse, blockading transport trucks and dropping banners from the slaughterhouse rooftop.

The protest coincides with footage released by Animal Liberation after activists secretly filmed pigs before they were killed. One activist climbed down into a 'chamber' while wearing an oxygen tank to position hidden cameras.