The male western lowland gorilla named Harambe was killed Saturday by a special zoo response team that feared the boy's life was in danger. Video taken by zoo visitors showed the gorilla at times appeared to be protective of the boy but also dragged him through the shallow moat.
Anthony Seta, an animal rights activist in Cincinnati, called the death "a senseless tragedy" but said the purpose of Monday's vigil wasn't to point fingers. Rather, he said, it was a tribute to Harambe, who turned 17 the day before he was shot.
WARNING: Some of the images in the video may be disturbing to watch:
The video captures a toddler's screams the moment he comes face-to-face with the giant gorilla.
"People can shout at the parents and people can shout at the zoo," Seta said. "The fact is that a gorilla that just celebrated his birthday has been killed."
Kim O'Connor, who witnessed the boy's fall, has said she heard the youngster say he wanted to get in the water with the gorillas. She said the boy's mother was with several other young children.
"The mother's like, 'No, you're not. No, you're not,'" O'Connor told WLWT-TV.
In the days since, people have taken to social media to voice their outrage about the killing of a member of an endangered species. A Facebook page called "Justice for Harambe" was created Saturday night, along with online petitions and another page calling for a June 5 protest at the zoo.
The zoo's director, Thane Maynard, said its dangerous-animal response team, consisting of full-time animal keepers, veterinarians and security staff, made the right call to kill the gorilla. He noted that the 400-pound-plus gorilla didn't appear to be attacking the child but was in an "agitated situation" and was "extremely strong." A tranquilizer wouldn't have immediately felled the gorilla, leaving the child in danger, Maynard said.
On Monday, he said the zoo had received messages of support and condolences from around the world. He said visitors left flowers at the gorilla exhibit and asked how they could support gorilla conservation.
"This is very emotional and people have expressed different feelings," Maynard said by email. "Not everyone shares the same opinion and that's OK. But we all share the love for animals."
The Gladys Porter Zoo in Brownsville, Texas, where Harambe spent most of his life, said Monday that its staff was deeply saddened by the gorilla's death.
Harambe was sent to Cincinnati less than two years ago in hopes he would eventually breed with females there.
The boy who entered into the exhibit was taken to Cincinnati Children's Hospital Medical Center for treatment and released hours after the fall. His parents said in a statement Sunday that he was "doing just fine."
Many social media commenters have criticized the boy's parents and said they should be held accountable. A Cincinnati police spokesman said no charges were being considered. A spokeswoman for the family said Monday they had no plans to make additional comments.
"I do think there's a degree of responsibility they have to be held to," said Kate Villanueva, a mother of two children from Erlanger, Kentucky. "You have to be watching your children at all times."
The People for the Ethical Treatment of Animals released a statement from its primatologist Julia Gallucci saying the zoo should have had better barriers between humans and the gorillas.
"This tragedy is exactly why PETA urges families to stay away from any facility that displays animals as sideshows for humans to gawk at," the statement said.
The zoo said that it's the first such spectator breach at Gorilla World since it opened in 1978 and that the exhibit undergoes regular outside inspections. The zoo said earlier this year that it planned to expand the exhibit.
Gorilla World remained closed Monday.