An "urgent inquiry" is needed after a male gorilla escaped from its enclosure at London Zoo, conservationists say.
Armed police were called in as staff sought to recapture the 29-stone gorilla, called Kumbuka, which was eventually tranquilised and taken back into captivity.
Despite Kumbuka only getting into the secure keepers' area, the Born Free Foundation said it was a "startling reminder" of the risks of keeping wild animals in captivity.
While the silverback's escape did not endanger the public, it "could have ended very differently", the foundation said.
The zoo said it was investigating the "minor incident" on Thursday.
The Born Free Foundation - which wants to see zoos phased out - has called on the Zoos Expert Committee, the government's advisory body, to investigate the welfare of great apes in zoos across the UK.
"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," said Chris Draper, the foundation's associate director for animal welfare and care.
"We are calling for an urgent inquiry into the circumstances surrounding this escape, and into safety procedures at London Zoo."
Visitor Brad Evans said customers had been locked inside the zoo's cafe about 5.10pm and told by staff a gorilla had escaped from its enclosure.
Metropolitan Police officers arrived at the zoo at 5.20pm and said the animal had been recaptured shortly before 7pm.
Eyewitnesses Hannah O'Donoghue-Hobbs and Charlotte Neild said the gorilla's escape was "like being in Jurassic Park".
'Up and grumbling'
Jonathan Mall, who was at a conference at the zoo, said he and other visitors were forced to take refuge in a bird enclosure for up to 30 minutes, as armed officers arrived.
"I was kind of scared, to be honest, because we were in a really closed space where everything is green and beautiful but there could be a gorilla hiding behind every bush," the 33-year-old, from Hamburg, said.
Malcolm Fitzpatrick, curator of mammals at London Zoo, told the BBC: "At 17.15 our male gorilla got out of his back dens into a secure keeper area. Our staff were able to respond quickly and Kumbuka was tranquilised and returned to his dens."
"We're happy to report that he's up and grumbling, and interacting with the rest of his family in his gorilla kingdom."
Kumbuka is understood to be one of at least seven gorillas at the zoo's £5m Gorilla Kingdom, which was opened in 2007 by the Duke of Edinburgh.
The recapture comes after Harambe, another gorilla, was shot and killed at Cincinnati Zoo in the US after a young boy fell into its enclosure.
The boy was not seriously hurt and Harambe's death sparked anger on social media.