Photos of shirtless workers clubbing pigs with bats in a Thai slaughterhouse have prompted campaigners to call for wider training and monitoring of humane welfare practices.

Undercover images taken in the central Thailand abattoir and shared with the Guardian also show workers stunning the animals on their eyes with what appear to be homemade stunning machines, contrary to equipment recommendations.

Although Thailand does have specific guidelines for pig slaughterhouses – which require that the animals be killed “without suffering” – humane slaughter practices in the Buddhist country’s small- and medium-sized abattoirs are virtually non-existent, say campaigners.