Mercy for Animals went undercover at Foster Farms in California
But hidden cameras recorded employees mistreating live chickens while hanging them upside down on a production line, hitting them and pulling out their feathers seemingly without reason.
One bird was seen boiled alive after surviving the blade meant to slit its throat and others were filmed after being crushed by lorries in a huge barn crowded with thousands of birds.
Baby chicks were seen being tossed out of crates on to the ground on one farm, with some being killed by the impact and others paralysed.
Mercy for Animals said some birds go through the slaughter process “completely conscious and able to feel pain”.
The group recorded the footage between April and June at a Foster Farms slaughterhouse in Fresno and the company's nearby farms.Chickens were filmed appearing to be punched and plucked alive while being hung upside down on the production line
Bob Barker, known as the host of the US version of game showThe Price Is Right, presented the film.
Saying the chickens had “miserable lives and painful deaths”, he added: “This is sickening cruelty and yet the meat from these animals is sold bearing the American Humane Certified label.”
The group’s petition calling on the watchdog to take action and “stop certifying blatant animal abuse” has so far reached almost 9,000 signatures.
Nathan Runkle, the founder and director of Mercy for Animals, criticised the American Humane Association’s standards as being too low to be labelled “humane” and called on it to ban the use of scalding tanks and blades, advocating painless gas instead.
“The humane-certified label is little more than a scam,” he said. “It dupes well-intending consumers into buying meat from factory farmed animals.”
Mark Stubis, a spokesman for the association, said the group has stringent standards and is dedicated to the humane treatment of all animals.
Baby chicks were unable to move after being thrown out of crates on to the ground
They were either directly involved in the abuse or failed to report it to management, the California-based company said.
A spokesperson added: “The behaviour of the individuals in this video is inappropriate and counter to our stringent animal welfare standards, procedures and policies.
“Since its founding in 1939, animal welfare has always been a top priority for Foster Farms.
“We believe raising chickens humanely is simply the right thing to do, and we take our commitment to humane values very seriously.”
Foster Farms said its employees receive annual animal welfare training on the proper handling of the birds and anyone violating policies can be disciplined or sacked.
Tony Botti, from the Fresno County Sheriff's Office, said the agency is investigating the allegations after receiving a complaint from Mercy for Animals last week.
Statistics held by the US Government show that hundreds of thousands of chickens are accidentally dropped alive into scalding tanks annually but only 0.008 per cent of all killed birds were found to have been "improperly slaughtered" least year.