Pigs suffer distress and struggle to breath and escape when gassed with carbon dioxide at abattoirs, RSPCA Australia says.
After a two-year investigation that collected video footage from the "gas chambers" inside Victorian abattoirs, a group from Animal Liberation Victoria temporarily halted production at the Diamond Valley Pork abattoir in Laverton on Monday.
Protesters stopped some trucks from delivering pigs and released videos showing animals violently thrashing and gasping inside the chambers.
In a statement, RSPCA Australia described the footage of an electric prodder used on pigs at an abattoir as "excessive and appalling", demonstrating the need for better training for abattoir workers and CCTV in all livestock slaughter areas.
Police arrived at Diamond Valley Pork about 4.40am on Monday.
Eight activists were removed from the roof and four from a "gas chamber" inside.
The activists' videos of pigs dying named Diamond Valley Pork and Australian Food Group, both in Laverton, and CA Sinclair in Benalla.
Animal Liberation Victoria was told of the alleged abuse two years ago.
One activist entered the on-site chambers wearing an oxygen tank and hid from cameras, an ALV statement said.
"The activist ... immediately noticed their eyes burning, which led to the discovery that when carbon dioxide reacts with liquids or mucus coated membranes it forms carbonic acid," the statement said.
"From their first lungful of gas, these pigs are burning from the inside out."
The RSPCA said carbon dioxide was the main pre-slaughter stunning method in the pig industry, was irreversible and less prone to human error compared to electrical stunning.
But it said carbon dioxide was "aversive and, in the 30 seconds it takes to lose consciousness, pigs will experience breathlessness and distress and will attempt to escape".
The RSPCA has urged the industry to use other gases and fast-track research into C02 stunning.
Police said the 12 activists would likely face trespass-related charges.
The abattoirs have been contacted for comment.
