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.”

The resistance movement is good news for Paris’s robust rat population. There is no formal count, a city hall official says. One rat-control expert’s estimate puts it at about four million, a 1.8 rat-to-human ratio.

Rat population numbers tend to be unreliable. The data that does exist suggests the Paris infestation could rival that of New York City’s, which has been estimated to be from several hundred thousand to several million.

Paris offers some key amenities for the rodents. It has a dense human population that, along with crowds of tourists, produces plenty of leftovers. Its ancient foundations date to the Roman Empire, providing the creatures extensive subterranean living space.

The problem, city officials say, is the rodents are asserting their place above ground en masse due to underground construction projects, the rising of the Seine River and people not disposing of rubbish correctly. “We don’t want to kill the entire rat population in Paris,” says the city-hall official. “There’s no point. We just want to control their expansion.”

Sightings of rats dining on garbage next to Paris’s monuments or darting along the Seine are commonplace.

“We really need to find a balance to live together,” says Claudine Duperret, a 59-year-old logistics manager who has a 600-member-strong Facebook group dubbed Rat-Prochement: Save the Rats that takes in homeless rats. Ms. Duperret lives with three—Mousty, Léon and Milou—she says were “rescued” from the streets.

Ms. Duperret and her cohorts will need to overcome centuries of European conflict between human and rat. In the Middle Ages, people were helpless to stop the creatures from invading pantries and destroying crops. Lacking effective poisons, authorities took to bringing legal charges against rats for their misdeeds, according to “The Criminal Prosecution and Capital Punishment of Animals,” a lengthy history by E.P. Evans.

The rats weren’t defenseless in such cases. When an ecclesiastical court in Autun, France, brought charges in the 16th century against a group of rats for destroying the local barley crop, a well-known lawyer named Bartholomew Chassenée was appointed by the court to represent them. Mr. Chassenée mounted a vigorous response.

“He urged, in the first place,” Mr. Evans wrote, “that inasmuch as the defendants were dispersed over a large tract of country and dwelt in numerous villages, a single summons was insufficient to notify them all.”

In the 20th century, scientists concluded that rats carrying fleas transmitted the plague in the Middle Ages, which killed up to 60% of Europe’s population. Though some scientists now doubt rats were to blame, the stigma has been difficult to shake.

This year, negative media coverage has taken its toll. In January, a Paris trash collector video recorded hundreds of rats swarming in a large garbage container by the Musée d’Orsay, one of which leapt toward him.

“It can’t go on like this,” he says in the video, which went viral after he sent it to the French newspaper Le Parisien. “It’s a huge plague.”

Ms. Duperret and other activists dismiss unease with the rodents as a form of “rat phobia.” Her rats live in cages and take turns roaming around her apartment.

Since she took in her first rat 11 years ago after her daughter wanted one as a pet, she has saved some 25, she says. She brings them with her on trips around the city and occasionally on holidays.

“I had a fantastic relationship with my first rat, Tycho,” she says. “He died of old age.”

Sometimes Ms. Duperret has difficulty finding the right spot to bury them. Socrates, a pet rat that died eight months ago, and three others now sit in her kitchen freezer, awaiting interment.

Most rats saved by Rat-Prochement members were raised as pets and then abandoned on the streets, the group says. Some are also albino rats that have escaped or been released from laboratories.

The 2007 Walt Disney animated film “Ratatouille,” the story of a country rat who becomes a Parisian-restaurant chef, generated a surge in children keeping rats as pets, Ms. Duperret says. But months later, many children lost interest and members of the group mobilized to take in the unwanted animals.

“Rats are not meant for children,” Ms. Duperret says. “They are very intelligent,” she added, referring to rats.

Some of Ms. Duperret’s allies have attempted to keep rats raised on the streets, with mixed results. Olga Bugni-Livolsi, a 49-year-old archivist, took in an injured rat and nursed it back to health. The animal, Tina, then destroyed two of its cages and finally retreated to the bathroom, where it chewed a hole in the wall.

Tina since died, and the incident hasn’t diminished her affection for the animals.

“We’re not asking people to love rats like us,” Ms. Bugni-Livolsi says. “What we’re asking is for people to reduce the number of rats without killing them.”

***Write to Matthew Dalton at Matthew.Dalton@wsj.com