A gorilla at a Cincinnati zoo was shot dead by zoo staff on Saturday when a 4-year-old boy fell into the animal's enclosure.
The killing of the gorilla triggered outrage and questions about safety, but zoo officials called the decision to use lethal force a tough but necessary choice.
The 181 kilogram gorilla was shot dead about 10 minutes after encountering and dragging the child.
By Sunday afternoon, a Facebook page titled "Justice for Harambe" had more than 3,000 likes.
More than 2,000 people signed a petition on Change.org sharply criticising the Cincinnati Police Department and the zoo for killing the animal, and called for the child's parents to be "held accountable for their actions of not supervising their child".
On Sunday Cincinnati police said the parents had not been charged, but that charges could eventually be sought by the Hamilton County Prosecuting Attorney.
The animal, named Harambe, was a Western lowland gorilla, an endangered species, and the zoo said it had intended to use him for breeding.
"If we think it's acceptable to kill a gorilla who has done nothing wrong, I don't think our city should have gorillas," Manvinder Singh posted on the Facebook page.
Witnesses told local television that the boy repeatedly expressed a desire to join the gorilla in the zoo habitat. Moments later, the boy crawled through a barrier and fell about 3.7 metres into a moat surrounding the habitat, where Harambe grabbed him, zoo officials said.
It was the first time in the 38-year history of the Cincinnati Zoo & Botanical Garden's gorilla exhibit that an unauthorized person was able to get into the enclosure, zoo president Thane Maynard said on Saturday.
"They made a tough choice and they made the right choice because they saved that little boy's life," he said, adding that a member of the zoo's Dangerous Animal Response Team fired the shot that killed the ape.
Maynard said the team decided to use deadly force instead of tranquilisers to subdue the gorilla because it could have taken some time for the drug to take effect when an animal was in agitated state.
One witness said she thought the gorilla was not hurting the boy but appeared to be protecting him.
Brittany Nicely told ABC News: "The gorilla rushed the boy, but did not hit the boy. He almost was guarding the boy, was protecting him."
Video footage shared on social media shows the gorilla dragging the boy through shallow water in the enclosure.
The child was taken to Cincinnati Children's Hospital Medical Centre for treatment of non-life threatening injuries.
Western lowland gorilla numbers in the dense rain forests of Cameroon, the Central African Republic, the Democratic Republic of Congo and Equatorial Guinea have declined by more than 60 percent over the last 20 to 25 years, according to the World Wildlife Federation.
The Cincinnati zoo was open on Sunday, although Gorilla World was expected to be closed indefinitely.