Panicked zoo visitors watched helplessly and shouted, "Stay calm!" while one woman yelled, "Mommy loves you!" as a 400-pound-plus gorilla loomed over a three-year-old boy who had fallen into a shallow moat at the Cincinnati Zoo.

The boy sat still in the water, looking up at the gorilla as the animal touched the child's hand and back. At one point, it looked as though the gorilla helped the youngster stand up.

Two witnesses said they thought the gorilla was trying to protect the boy at first before getting spooked by the screams of onlookers. The animal then picked the child up out of the moat and dragged him to another spot inside the exhibit, zoo officials said.

Fearing for the little boy's life, the zoo's dangerous-animal response team shot and killed the 17-year-old ape, named Harambe.

The child, whose name was not released, was taken to Cincinnati Children's Hospital Medical Center, where he was expected to recover. Hospital officials said they couldn't release any information on him.

Zoo Director Thane Maynard said the gorilla didn't appear to be attacking the child but was "an extremely strong" animal in an agitated situation. He said tranquillizing the gorilla wouldn't have knocked it out immediately, leaving the boy in danger.

"They made a tough choice and they made the right choice because they saved that little boy's life," Maynard said. "It could have been very bad."

Twitter reaction quickly followed with the hashtag #RIPHarambe. Many were outraged that the animal had been killed.

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Zoo officials said the three-year-old climbed through a barrier at the Gorilla World exhibit and fell more than three metres into the moat Saturday afternoon. He was in there for about 10 minutes. Two female gorillas also were in the enclosure.

One witness said that just before the boy fell, she saw him in bushes beyond a fence around the exhibit.

Happy 17th birthday to silverback gorilla Harambe! <a href="https://t.co/FeITuw0hPb">https://t.co/FeITuw0hPb</a> <a href="https://t.co/FrfWUSKacV">pic.twitter.com/FrfWUSKacV</a> —@CincinnatiZoo

"I tried to grab for him. I started yelling at him to come back," Brittany Nicely told The Cincinnati Enquirer. "Everybody started screaming and going crazy. It happened so fast."

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Witness Kim O'Connor shared video she and her family recorded with WLWT-TV of the boy and Harambe. The two appear in a corner of the exhibit while visitors yell, "Somebody call the zoo!" and "Mommy's right here!" The station did not air portions of the video showing the gorilla dragging the boy.

Zoo staff cleared the area and visitors heard gunfire a few minutes later. Firefighters then rushed into the enclosure and picked up the boy.

Lt. Steve Saunders, a Cincinnati police spokesman, said there are no plans to charge the parents.

Maynard called the killing a tragic death of a critically endangered species and a huge loss for the zoo and the gorilla population worldwide.

Harambe came to Cincinnati in 2015 from the Gladys Porter Zoo in Brownsville, Texas.