The shooting of a 400-pound gorilla who grabbed a four-year-old boy after he entered his enclosure has sparked public outrage with thousands of people calling for "justice" for the animal.

A petition entitled 'Justice for Harambe' and a Facebook page of the same name, both call for the boy's parents to be held responsible for the death of the gorilla which online critics are blaming on "the lack of supervision" of the child.

The petition reads: "This beautiful gorilla lost his life because the boy's parents did not keep a closer watch on the child.

"We the undersigned believe that the child would not have been able to enter the enclosure under proper parental supervision."

The change.org petition, which was set up on Sunday, had more than 53,000 supporters by Monday morning with signatories appearing to agree the zoo did the right thing to save the boy but questioning how he had been able to get into the silver back's enclosure in the first place.

It comes after 14-year-old Harambe was shot by an employee at Cincinnati zoo as he stood with the young boy between his legs after scooping up the toddler just moments after he crawled through a barrier and into the animal pen.