Kerala's proposed culling of stray dogs "is based on a misreading of the law and will likely be found illegal", the group's managing director N.G. Jayasimha said in a statement.
He said it would be a violation of both constitutional and statutory law.
"The constitution protects the rights of animals and humans to be treated with dignity and compassion," he said.
"In addition, the Prevention of Cruelty to Animals Act, 1960, includes Animal Birth Control (ABC) rules mandating that stray dogs are to be sterilised, vaccinated and subsequently released into the same area..."
"When dealing with stray dogs, the Prevention of Cruelty to Animals Act, 1960 and the ABC Rules hold the field over the provisions of the Kerala Municipality Act, 1994 and must be given full effect.
"It would be illegal to deal with stray dogs in manner that is inconsistent with the ABC Rules. Any measures to undertake mass culling of stray dogs would be unlawful."