The Canadian Horse Defence Coalition (CDHC) says the footage shows that both facilities fail to meet humane slaughter standards.
The coalition was responding to the reported comments of Bill desBarres, chairman of the Horse Welfare Alliance of Canada (HWAC).
DesBarres was reported in The Western Producer magazine as saying the footage was most likely a fabrication.
"I'm not convinced that those pictures were taken at Fort Macleod," he said.
However, the CDHC's eastern region director, Shelley Grainger, said of the footage: "The evidence provided compelling proof that puts into question the effectiveness of the assembly-line slaughter of horses.
"The evidence demonstrates that both the facilities in Alberta and Quebec fail to meet humane slaughter standards."
The footage, she argues, is incontrovertible. In a court of law it would stand the test of cross examination as evidence.
"The evidence obtained on the footage at both facilities was random - these instances we see of horses suffering terribly happened on those random days, and we can be certain they happen every day.
"As there are only two government-approved methods of stunning horses in Canada - captive bolt gun and rifle - and both methods have been shown to cause prolonged suffering in horses, it is evident that these animals cannot be slaughtered humanely in today's slaughterhouse assembly lines," Grainger says.
DesBarres was reported by The Western Producer as saying: "These things are produced by people who have a different agenda ... [They are] against animal agriculture, period."
The Equine Welfare Alliance (EWA) labelled DesBarres a leading slaughter proponent and called his fabrication suggestion outrageous.
It said that DesBarres, faced with "absolute evidence of the true nature of the slaughter process", had opted for the "Nixon Gambit" in suggesting the footage was fabricated.
"One has to wonder," said the EWA's John Holland, "does desBarres believe the CHDC tortured horses in staged mock-ups of the plants or does he think they hired Steven Spielberg to produce a digital fantasy?"
The EWA said the video revelations are problematic for the Canadian Food Inspection Agency, which has yet to respond publicly to findings of a study published recently in the journal Food and Chemical Toxicology showing that horses given the banned carcinogen Phenylbutazone had routinely slipped through the inspection process and been slaughtered for human consumption.
Grainger noted that, from July 31, all slaughter-bound horses in Canada would require an equine identification document (EID) verifying they had not received phenylbutazone, clenbuterol, or various other drugs and steroids. As well, there is an extensive list of medications requiring a six-month withdrawal period.
The controversy around the video comes as bills have been introduced into several US state legislatures in support of horse slaughter.
"They are just ignoring reality," says the EWA's Vicki Tobin, "They can't keep ignoring the drug concerns, the horrific conditions at the slaughter plants and the cruelty inherent with the entire slaughter pipeline. The only states that want horse slaughter are those who have never experienced it."
The CDHC's video footage prompted a call by the Humane Society of the United States to ban the export of horses to Canada for slaughter.
The World Society for the Protection of Animals said it was "appalled by evidence of grossly inhumane slaughter of horses taking place in Canada," and called for immediate action by authorities.
One noted veterinarian who watched the footage labelled one plant an equine house of horrors.
Equine Canada said it had contacted the Canadian Food Inspection Agency over the material.
The Horse Welfare Alliance of Canada, in a statement by DesBarres to its partners and associates, said the organisation was not in a position to authenticate the footage or the content.
"We have expressed to the Canadian Food Inspection Agency and its minister our concern and asked that they take appropriate steps to review this matter thoroughly.
"That said, we note that the facilities are fully regulated and regularly inspected by the CFIA. Their work has always been and continues to be integral to the high standards Canada has for the humane treatment of horses.
"Enhancing public awareness of the equine industry is one of our major objectives. Obviously, we become very concerned when accusations are made about our industry.
"However, the mechanisms of our system ensure any deviations from our high standards are dealt with swiftly. Our track record speaks for itself in taking the necessary steps to ensure the best management of horses in Canada."