DISTRESSED prep students were crying as they witnessed the horrific beating that killed a gorilla at Melbourne Zoo.

Julia, a 33-year old gorilla, died at 3am on Sunday from complications that developed from injuries inflicted by young silverback gorilla Otana inside the enclosure, zoo spokeswoman Judith Henke said in a statement.

Mother Belinda Carroll said 18 students from Altona Green Primary School, including her daughter, Bella, 5, were touring the zoo on Friday and visited the gorilla enclosure at 9.20am when the attack took place

“(The noise from the slapping) was really loud and her lower fur was all bloody and red and you could see a gash,” she claimed.

“Julia would run away and then the male would come back and pounce on her, slapping her again. It was really frightening.”

Ms Carroll said the preps had been excited at first to see the gorillas, but that quickly turned to shock and tears when they realised Julia was being hurt.

She alleges a zoo volunteer and two other uniformed workers had witnessed the ­incident.

In a statement, Ms Henke said the zoo was mourning Julia’s death and that although she had been unwell, she had not displayed any wounds.

“Staff monitoring the group as usual throughout Friday observed Otana displaying an unexpected level of aggression towards Julia,” Ms Henke said.

“Silverbacks assert their dominance as a normal part of behaviour, (but) in this instance, the results of the aggression Otana displayed towards Julia ... were unexpected and unprecedented.”

Julia spent part of Friday night sleeping in one of the enclosure’s heated caves away from the other gorillas.

She appeared to be unwell the following morning and was moved to the veterinary hospital, where it was discovered she had experienced “extensive trauma and significant complications” from the attack.

A vet team worked into the early hours of Sunday morning to stabilise Julia, but she could not be saved, the statement said.

Otana, 13, has been separated from the group. He arrived at Melbourne Zoo in September 2013 from the UK.

Julia came to Melbourne in 1997 from the Jersey Zoo.

“Staff throughout the zoo are devastated and the primate keepers have been offered counselling to help them through this difficult time,” Ms Henke said.

samantha.landy@news.com.au