PARIS—Rats were popping up at supermarkets, parks and nurseries when a city official convened a crisis meeting last fall to discuss ways to cull the population.

That was the first time Geoffroy Boulard, mayor of the 17th arrondissement in northwestern Paris, realized the rodents are backed by a vocal lobby. Ten protesters stepped forward to denounce exterminators’ plans to poison the animals. They urged a more humane method: Deploy birth-control drugs.

Their position was “indefensible, given the scale of the infestation,” says Mr. Boulard. “We can’t get accustomed to having rats in public spaces.”

The city’s pro-rat activists disagree. Rattus norvegicus, the species of rat endemic to big cities, has the right to inhabit the City of Light like any other mammal, they say. The activists regard poisons and rat traps as a form of unusual cruelty. When the city stepped up exterminations 18 months ago, they unleashed an online petition that garnered almost 26,000 signatures.

“We are very disturbed,” says Jo Benchetrit, a retired psychologist who created the petition to save the rats. The defense of rights for rats is only seen as “abnormal,” she says, ”because others are able to live among the banality of such cruelty.”