The distressed 911 call of a mother whose four-year-old son entered a gorilla's cage on Sunday has been released.

In her horrified conversation with emergency services, Michelle Gregg can be heard frantically responding to the handler's request for an address: "My son fell in with a gorilla, there's a male gorilla standing over him".

"I need somebody to contact the zoo, please".

When the responder asks how old her child is, Gregg can be heard shouting "be calm" to her son.

She then screams: "He's dragging my son, I can't watch this".

Later she pleads: "Hurry... Hurry... the gorillas are out".

Another witness eventually takes the phone from Gregg.

When asked if there is a zookeeper nearby, the witness responds: "Oh God, it's got his pants. He's taken the baby. He's taken the baby into the cave".

Somebody is then heard shouting "everybody out".

Harambe, a 17-year-old silverback gorilla, was shot shortly after by zoo workers.

The animal's death sparked sparked public outrage with thousands of people calling for "justice" for the animal and blaming the boy's mother for the "lack of supervision" of her child.