A vigil has been held at Cincinnati Zoo in memory of a 400-lb gorilla fatally shot after a four-year-old boy fell into its enclosure.

Cincinnati Zoo's special response team shot Harambe, the 17-year-old African western lowland gorilla, when it dragged the boy out of a moat after he fell climbing through a public barrier at the Gorilla World exhibit on Saturday afternoon.

The child was taken to hospital after the incident, but suffered no serious injuries according to a Facebook post written by his mother.

Harambe (pictured) was a 17-year-old silverback western lowland gorilla (Reuters)

The zoo’s director, Thane Maynard, has defended its decision to kill the gorilla, saying a tranquilier wouldn't have immediately felled the creature, who was strong enough to crush a coconut, leaving the child in danger.

Mr Maynard said Harambe was agitated, disorientated and acting erratically at the time of the incident.

"This is very emotional and people have expressed different feelings," Mr Maynard said by email. "Not everyone shares the same opinion and that's OK. But we all share the love for animals."

Dozens of people, including animal rights activists, attended Monday’s tribute to Harambe, who turned 17 the day that he was shot.

Anthony Seta, an animal rights activist in Cincinnati, called the death "a senseless tragedy" but said the vigil’s aim was not to place blame on any of the involved parties.

Cincinnati zoo gorilla shot dead as boy falls into enclosure

"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."

The boy’s mother, Michelle Gregg, has faced mounting controversy over the incident and has reportedly recieved a police warning she could face charges.

Ms Gregg has defended herself against critics on Facebook saying: “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.”