Photographs released by the protest group on Thursday morning show people sitting inside machinery at the Firth Road meatworks, in Benalla, Victoria.

They are protesting at a carbon dioxide chamber used in the slaughtering process, with the trespassers chained to equipment and demanding the end of its use.

A woman is also reportedly on the roof of the abattoir, and an ambulance has been called to treat a 25-year-old woman for shock.

It's believed the group has been in the area for three days.

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A statement from the Farm Transparency Project says 30 people are inside the site, with some blocking the race to stop pigs from being killed.

Director Chris Delforce said he was locked inside a chamber and had filmed the process and "can't wait any longer for someone else to act".

"Those involved have said they are willing to stay for as long as possible to draw attention to this issue and push for a total ban on gas chambers," the statement said.

Police, including officers from Benalla and Wangaratta, are at the scene with paramedics.

The business has been targeted in similar protests multiple times in the past.