Visitors to London Zoo were "never in any danger" after a large silverback gorilla escaped from his enclosure.
Armed police were called to the central London attraction and visitors were evacuated as keepers desperately tried to recapture the loose animal.
But Malcolm Fitzpatrick, curator of mammals at the zoo, described the escape of Kumbuka as a "minor incident" in which the male gorilla got into a secure keeper area that was not open to the public.
Mr Fitzpatrick said the western lowland had been tranquillised before being returned to the Gorilla Kingdom, where he was "up and grumbling and interacting with the rest of his gorilla family".
He added: "It was a safe, secure keeper area, so at no time did he actually get out into the zoo."
The incident is now under investigation, he said.
Mr Fitzpatrick told reporters Kumbuka was in the keepers' area for more than an hour until vets were able to tranquillise him.
He said: "At 5.15pm this afternoon our male gorilla got out of his back dens into a secure keeper area.
"Our staff were able to respond quickly and Kumbuka was tranquillised and returned to his dens, where I am happy to report he is up and grumbling and interacting with the rest of his gorilla family in Gorilla Kingdom."
Mr Fitzpatrick would not confirm if there were keepers in the area at the time and said the public gorilla viewing zone had only a "handful" of people there then.
He added: "At no time were any of our visitors in any danger. The gorilla did not get out of the safe space, there were only about 100 visitors, it was the end of the day and I would like to thank all of those visitors for co-operating and moving in to buildings."
Mr Fitzpatrick said keepers would be staying on into the evening to check up on Kumbuka and giving him his "favourite treats".
A statement from ZSL London Zoo said the 18-year-old gorilla was contained within the exhibit's off-show area, and that an investigation would be carried out in to how he escaped.
A spokeswoman said: "The exhibit is secure and we are grateful to all of our staff and visitors for their co-operation, enabling us to resolve the situation quickly and efficiently. ZSL London Zoo will be open tomorrow."
The zoo was immediately put on lock-down after Kumbuka escaped, with visitors being ordered to shelter inside any buildings they could.
Visitors to the gorilla attraction said they saw the "lead male" apparently agitated and charging at the enclosure's glass walls moments before the escape.
Neuropsychologist Dr Jonathan Mall was at the Weird and Wonderful World of Field market research conference when the incident happened.
Dr Mall, one of the speakers at the conference, had gone for a walk during a break in the programme and was next to the gorilla enclosure when a siren went off at around 5.20pm, and a call over the speakers asked for staff to urgently go to the site.
The 33-year-old, from Hamburg in Germany, who was representing his company Neuro Flash, told the Press Association he saw two members of staff "frantically running around" in the enclosure, and when he jokingly asked one if a gorilla was loose they said: "I don't know, please go outside the area."
He added: "Another distressed staff member said 'Please move quickly and go inside a building', and she told us to leave, I saw other people running for buildings."
Dr Mall and other visitors were forced to hide inside a bird attraction, stranded for around half an hour while staff brought Kumbuka under control.
He said: "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.
"Any sound I heard scared the shit out of me."
Eventually staff went from building to building to rescue visitors and evacuate them from the zoo.
Dr Mall said he saw at least 20 people carrying large guns and that the police helicopter arrived within five minutes.
He also said another speaker at the conference had seen one of the gorillas acting "really agitated" in its enclosure around 15 minutes before the escape, saying it had repeatedly run towards the glass walls and hit it as people took pictures of it.
Jonny Briers, 22, told the Evening Standard: "We were at the gorilla enclosure and the gorilla charged at the glass. It didn't break it, we saw it do it earlier in the day as well.
"And then we started to leave and we heard the siren go off. Then we saw zookeepers running and they told us to go inside. We went into the aquarium and they locked the doors."
According to the zoo's website Kumbuka is one of at least seven gorillas living in its Gorilla Kingdom.
Warnings that animals could escape
The £5 million Gorilla Kingdom was opened by the Duke of Edinburgh in 2007, then the largest investment at the zoo for 40 years.
The same year there were warnings that animals could escape unless security was improved.
A report said that although it had no concerns about animal enclosures, in the circumstances of a dangerous animal escaping it was "unlikely" the existing perimeter fence would be sufficient to contain dangerous animals.
It warned that if they escaped there would be nothing to stop them roaming free, and said marksmen with tranquilliser darts would have little time to react.
The year before, 12 squirrel monkeys managed to escape their enclosure.
In May a gorilla was shot dead by keepers after it grabbed a four-year-old boy who fell into a moat at a US zoo.
Harambe, a 17-year-old, 400-pound-plus male western lowland, was killed after he dragged the youngster around for 10 minutes after he fell 12 feet into the exhibit at Cincinnati Zoo.
Zoo officials made the decision because they felt the boy was in a "life-threatening situation".