A leading wildlife group has said that the escape of a large silverback gorilla from its enclosure at London Zoo "could have ended very differently", and there must be an urgent inquiry into it.
The Born Free Foundation said the incident was a "startling reminder" of the risks of keeping dangerous wild animals in captivity.
Visitors to the popular attraction described fearing for their safety as they were ordered to take cover in buildings when the 29st dominant male ape got out of its den on Thursday evening.
Armed police were deployed at the central London zoo as keepers desperately tried to locate the animal, described by witnesses as "agitated", which was eventually tranquillised and returned to its enclosure.
A zoo official said members of the public were "never in any danger" as the western lowland gorilla, called Kumbuka, had remained in a secure keeper area.
An investigation has been launched into the "minor incident", they added.
However, the Born Free Foundation has called for an inquiry and for the Zoos Expert Committee, the Government advisory body, to investigate the safety and welfare of great apes in UK zoos.
Chris Draper, associate director for animal welfare and care at the foundation, said: "While we are relieved that this incident apparently ended without injury to visitors or to the gorilla, it is yet another startling reminder of the risks associated with maintaining dangerous wild animals in captivity.
"This incident could have ended very differently. We are calling for an urgent inquiry into the circumstances surrounding this escape, and into safety procedures at London Zoo."
The Born Free Foundation, which campaigns for zoos to be phased out, called on the Government to increase penalties for attractions if they are found to have put visitors or animals at risk.
There were no reports of injuries in Thursday's incident and Kumbuka was later seen "up and grumbling and interacting with the rest of his gorilla family", said Malcolm Fitzpatrick, curator of mammals at the zoo.
According to the zoo's website Kumbuka is one of at least seven gorillas living in its Gorilla Kingdom.