The protesters said the dogs are innocent, and many claimed the victims were probably killed by humans. They acknowledged the famished dogs that live in a hilltop park in an east-side slum where the bodies were found may have bitten the corpses after they were already dead.
"Dog friends, the people are with you!" the protesters chanted, as well as, "The dogs aren't criminals, the police are inept!"
"We are completely certain ... the dogs are innocent," said Nominis de Esparza, an animal activist who has adopted 30 cats.
Autopsies determined that the three women, a teenage boy and a baby found in the park since mid-December died of loss of blood due to bites from multiple dogs.
But those findings have been met with widespread skepticism in a country where drug gangs frequently dump bodies of their victims in public spaces, and prosecutors seldom thoroughly investigate such crimes. The idea has taken hold among many that killers dumped the bodies in the park, hoping that packs of stray dogs would destroy the evidence.
"This was a crime committed by humans, for a settling of accounts or who knows what," said De Esparza, using the Spanish word "ajuste" frequently employed to describe drug gang killings.
Tests on the dogs have so far been inconclusive.
The city prosecutor's office said initial tests on the first 25 strays gave no indication they ate human flesh. An employee of the city prosecutors' office, who was not authorized to be quoted by name, said almost no food of any kind was found in the dogs' stomachs, much less human flesh. But he said officials were still awaiting results from tests on the dogs' fur and paws to see if any human DNA was present.
Jose Luis Carranza, of the Citizens Front for Animal Rights, criticized city authorities for ordering round-ups of strays in the aftermath of the killings. Carranza said protesters want the raids stopped because only animal control officers are allowed to seize dogs in Mexico City, and only on specific complaints involving individual animals.
"If the authorities really want to crack down on the overpopulation of dogs, then they should go after the clandestine puppy sellers," Carranza said. "Every day there are people selling dogs on the streets, and the police don't do anything."
The 57 mutts rounded up at the Cerro de la Estrella park, where the attacks occurred, include a few about the size of a Labrador, but many are small or mid-size dogs, including beagle and border-collie mixes. Twenty-three are puppies or very young dogs.
Many look like the discarded pets they are. Residents near the 353-acre (143-hectare) park in the poor Iztapalapa neighborhood say people regularly drop off unwanted pets there, but say the dogs have never caused problems before.
Moises Heiblum, professor of animal behavior at the school of veterinary medicine at the National Autonomous University of Mexico, said feral dogs as individuals "probably could not carry out a ferocious attack of this type" and normally avoid human contact.
But the dynamics change when a pack is formed, Heiblum said. "When a group comes together, they are capable of an extremely intense and even fatal attack. That is possible."
Animal control warden Armando Garcia, who was patrolling with an assault rifle this week, said there was no question that strays had formed a pack in at least one part of the park.
"You can tell when there's a pack: There's an alpha dogs and his followers, and they've marked out territory and they challenge you when you enter it, with growls and barking," Garcia said.
On Friday, authorities in Iztapalapa announced that the dogs taken into custody would be put up for adoption. They had earlier promised animal rights groups that the dogs would not be killed.
The dogs will get shots, special baths and medical treatment before being given away.