Professor Gisela Kaplan, who has worked with great apes, told radio station 3AW the 200 kilogram "gentle giant" was carrying the boy as if he was its own child.

She said Harambe was likely startled by screaming onlookers and was trying to drag the boy away.

"Gorillas don't attack. They are a peaceful species," she said.

"They will only attack if there's a real threat to their own existence.

"There are records of them saving children before in very similar situations when a child had fallen into an enclosure."

However, Professor Kaplan agreed the child's life rightfully took precedence over the animal's.

Animal lovers throughout the world have been outraged over the Cincinnati Zoo's killing of Harambe.

The gorilla dragged the boy through shallow water and up a rock wall on Saturday.

Professor Kaplan told 3AW there could have been a peaceful ending to the situation.

"[Harambe] was probably doing all the kind of benign things that a gorilla can do and he was being penalised for it and lost his life and traumatised his entire family," she said.

"Why couldn't they have shot him in the shoulder?"

However, Cincinnati Zoo director Thane Maynard on Monday stood by the decision to shoot dead the gorilla.

"This child was being dragged around. His head was banging on the concrete," he said.

"This was not a gentle thing. The child was at risk. We're very fortunate that he was OK."

Mr Maynard said tranquillisers were not an option because they could have a delayed effect and cause the situation to escalate.

"The gorilla was clearly agitated," he said.

"Looking back, we would make the same decision."

More than 200,000 people have signed online petitions on Change.org protesting the shooting of the Western lowland gorilla, whose species is listed as endangered.

Some petitions urged police to hold the child's parents accountable.

Michelle Gregg, who identified herself on Facebook as the boy's mother, said her son was recovering from a concussion and a few scrapes.

"I want to thank everyone for their thoughts and prayers today. What started off as a wonderful day turned into a scary one," she said on Facebook.

"For those of you that have seen the news or been on social media that was my son that fell in the gorilla exhibit at the zoo. God protected my child until the authorities were able to get to him. My son is safe and was able to walk away with a concussion and a few scrapes ... no broken bones or internal injuries.

"As a society we are quick to judge how a parent could take their eyes off of their child and if anyone knows me I keep a tight watch on my kids. Accidents happen but I am thankful that the right people were in the right place today."

She also expressed condolences to the zoo for the loss of 17-year-old Harambe.

The killing of a 33-year-old gorilla at Melbourne Zoo last year also prompted outrage.

Julia was savagely beaten by a dominant young male silverback called Otana.

Animal rights group PETA said the zoo could have done more to prevent the death with closer monitoring and immediate action after the attack.

- with Reuters