After attempting to make a dramatic bid for freedom, Kumbuka the gorilla has now been safely returned to his enclosure at London Zoo.
Zookeepers were today seen boarding up the glass partition which cracked as the 'agitated' gorilla escaped from its den - through a door which may have been left open.
Male silverback Kumbuka, an 18-year-old, 29-stone western lowland gorilla, escaped the enclosure yesterday evening for 90 minutes.
Visitors were said to have been taunting the gorilla, prompting social media users to voice their sympathy for the animal, writing: 'We all would get nuts if we were locked in a cage and people would stare at us every day. Poor creature.'
Staff said Kumbuka was given a chocolate chip cookie yesterday evening as a 'special treat' from his specially managed diet tailored to his nutritional needs.
The incident comes amid warnings from one zookeeper, who allegedly told BuzzFeed news: 'He's a f*****g psycho, that ape. He's smacked the enclosure glass a couple of times.'
It was initially reported the animal may have smashed the glass partition surrounding its indoor den - but now it has emerged a door may have been left open.
A source told the Evening Standard: 'Every enclosure has a gated area which is off-limits to the public to stop animals running straight past when the door is opened. That door shouldn't have been left open.'
Opened in 2007 by the Duke of Edinburgh, the zoo's £5.3 million Gorilla Kingdom is home to six primates, with 45mm-thick panes of glass forming a 5m barrier between zoo visitors and gorillas.
One social media user wrote: 'I feel sorry for the gorilla. If I was in a glass case with people taunting and staring at me, I'd charge at them too.'
Another added: 'People were screaming and egging him on. No wonder he wanted to escape,' while another tweeted: 'If you were trapped behind glass with people gawping at you everyday, you would make a bid for freedom!'
One member of the public called the incident 'poetic justice', adding: 'The poetic justice of people being locked up as gorilla romps around zoo is brilliant!'
The irony was echoed by others, who added: 'locking the humans inside while the gorilla roams around - the irony'.
The Born Free Foundation said the incident was a 'startling reminder' of the risks of keeping dangerous wild animals in captivity.
Leading primate expert Professor Phyllis Lee today said Kumbuka should not be put back on show to the public and should be moved to another zoo because he is 'unhappy in his environment', adding his break-out signals 'crisis and desperation'.
Zoo curator Malcolm Fitzpatrick today refused to explain how Kumbuka escaped when asked six times during a live BBC interview.
He told the Today programme: 'Categorically I can say that our male silverback gorilla, Kumbuka, didn't break through the glass'.
Dan Bourke, a journalist at The Mirror, said 'something has not felt right at Gorilla Kingdom recently', adding: 'we've noticed an increased agitation in Kumbuka'.
Mr Bourke suggested the gorilla may have been furious because he felt his children were threatened, and commenting on an incident last week, said: 'Kumbuka started moving quicker and quicker from left to right.
'As the noise increased so did his pace until in a frenzy he bashed the reinforced window then his chest.'
John Davis, 56, who visited Gorilla Kingdom with his two-year-old son Eden, told the Evening Standard: 'I go there pretty much every day and know the gorillas well.
'Most of the time the big one just sits and stares but yesterday he was looking really angry.
'People were banging on the glass and taking pictures using their flashes.
'Then he lurched towards us and smashed against the glass. My little one was so scared he ran away in tears.'
Visitors to the popular attraction were ordered to take cover in buildings when the 7ft dominant male ape got out of its den on from 5.15pm-6.20pm on Thursday evening.
A spokesman for London Zoo this morning said Kumbuka was treated by keepers overnight who stayed to observe him.
The zoo said he had returned to the main enclosure and an investigation was 'ongoing', adding: 'The gorilla remained contained within the exhibit's off show area.'
Eyewitnesses told MailOnline they were warned not to look the seven-foot tall gorilla in the eye for fear of agitating him - just minutes before he launched the audacious bid to free himself yesterday afternoon.
Despite the warnings, some visitors were heard shouting at Kumbuka and 'egging him on' in the seconds before he charged - in scenes described as 'like something out of Jurassic Park'.
Armed police and a force helicopter were scrambled to the scene and officers - wielding semi-automatic guns - desperately tried to track him down while visitors were locked away for their own safety.
Members of the public were ordered to retreat to safety and were in lockdown in sites across the zoo - with many posting pictures from inside a butterfly enclosure and a canteen area.
Kumbuka was later traced by police and zoo staff and tranquilised by vets. He is now said to be 'awake and well' in his enclosure and the zoo will reopen today as normal.
However, despite the incident being resolved without harm to any visitors or animals, concerns have been raised about the safety of the gorilla enclosure.
It has since emerged that zoo visitors had previously been warned of damage to the perimeter glass of the enclosure in the weeks prior to the incident.
Chris Draper, Associate Director for Animal Welfare and Care at the Born Free Foundation charity, 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.'
Veterinary student Chloe Hughes was on her first visit to London Zoo when she filmed Kumbuka charge towards her.
The 23-year-old student, from Surrey, told MailOnline: 'I was standing near the front [of the enclosure]. We were in there for a long time. The silverback was distressed. He was looking at everyone.
'Staff asked everyone to be quiet. We were at the front and they asked us to get out. They came to the front with their arms out.
'He seemed to stare at one spot for a while. People were screaming and egging him on.
'Then he jumped onto the rope and smashed against the glass. It was very scary.
'After that happened, we all screamed.'
She said there was panic after Kumbuka charged at the glass as people - including parents pushing prams - stood dangerously close on the other side.
'We heard the panic in the zoo keepers' voice,' she said. 'They were shouting at us to get out.
'I don't think I would go back to a zoo again. I'm not a big fan of zoos. I've heard a lot of good things about London Zoo, but it makes you think.'
Oliver Barker, 24, who was at the zoo for a marketing conference, told MailOnline that the gorilla had seemed 'agitated' earlier in the day, and keepers had warned visitors not to stare at him.
He said the animal looked 'intimidating' - and a colleague told him the gorilla had charged at the glass shortly before the escape.
He said: 'He looked like he was in a sort of trance, he definitely wasn't relaxed at all.
'He looked very intimidating. The zookeeper came over and said, 'don't aggravate him by staring at him'.
'She could tell he was very agitated, and lo and behold soon after we were told he had escaped.'
Mr Barker said a colleague went into the enclosure soon after he had been in there, and saw the animal charging.
He said: 'He said he charged at the glass but the glass didn't even wobble.
'Five minutes after that we were told to get into the nearest safe room.'
A spokesman for London Zoo said: 'A male gorilla got out of its pen into a non-public keeper area at 5.13pm.
'He remained contained in the exhibit's off-show area where staff responded immediately.
'It was our 18-year-old gorilla Kumbuka and he was tranquilised by vets and returned to his pen.
'I can confirm that he is awake and well, and the zoo will be open today.'
Malcolm Fitzpatrick, the zoo's curator of mammals, said there were no injuries to visitors or zoo workers, adding that Kumbuka recovered quickly.
'I'm happy to report he's actually up and grumbling and interacting with the rest of his gorilla family.'
He 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 told reporters Kumbuka was in the keepers' area for more than an hour until vets were able to tranquillise him.
He 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'. The incident is now under investigation, he added.
Scotland Yard said an incident involving an escaped gorilla at London Zoo had 'concluded'.
Eyewitness Brad Evans earlier told BBC Radio London: 'We were in the zoo for the day, having a cup of coffee in the main restaurant area when they locked us all in and said there was an incident.
'They gave us free teas and coffees and obviously we were asking what was going on and they told us that a gorilla had got out of its enclosure and that we weren't allowed out of the park at half-five so we had to wait.
'As we were waiting we saw the police turning up in numbers with loads of guns.'
Hannah O'Donoghue-Hobbs and her friend Charlotte travelled from Manchester for a conference and were locked-in during the incident. They videoed their experience and posted it online. She said: 'It was just like a scene from Jurassic Park.'
She said she saw about 30 armed officers.
The 24-year-old told MailOnline: 'Everyone was all over the zoo, we were told to get into the nearest building.
'The staff were saying there was a zoo emergency, they didn't give us any information but I heard through the staff walkie-talkie that it was a gorilla.
'I didn't believe it at first, but then we saw more and more police, there were around 30 armed police, I hadn't seen police like that before.
'We saw the police coming in with big guns, and there were people from the zoo with guns.'
She said visitors were told to stay away from the windows and weren't allowed to take photographs.
Rob Hogan, 37, was visiting a marketing conference and saw the gorilla just seconds before he escaped.
He said: 'We were at the conference all day and it broke for tea time and a group of us went for a walk to see the gorillas.
'We were looking at the silverback and he suddenly jumped at the window.
'It was like a car hitting the window, it was really scary.
'A warden suddenly started ushering us into the reptile room, there was about 20 of us.
'After about an hour they said it was contained and took us out of the park into the street.
'All our stuff was still inside though and now the gorilla has been caught there's a live band playing and some of my colleagues are in there drinking beers.
'The staff said he had got somewhere he shouldn't be, but not got out.'
Former England U21 and Leicester Tigers prop Matt Hampson, whose foundation was set up to inspire and support young people seriously injured through sport, tweeted: 'Well that's the first time that has happened, was doing a talk at London Zoo this evening but evacuated because of an escaped Gorilla!'
He added: 'We were escorted into the zoo at about 5pm and we parked outside the enclosure and I was getting out my car and heard a really loud alarm going off.
'There was lots of keepers and people running about and I was told to go into the room because an animal had escaped.
'They didn't say what it was.
'We went into room where I was going to be giving the talk and we were locked in there for about an hour.
'One of the keepers came to the door and said a gorilla had escaped and that everybody was going to have to leave the premises.
'I put had some bananas in my bag and made light of the situation, I said maybe the gorilla would like to eat the bananas?
'It was quite scary, initially I was sort of c******* myself.'
Neuropsychologist Dr Jonathan Mall was also 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, said 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.'
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 s*** out of me.'
Dr Mall said he saw at least 20 people carrying large guns and that the police helicopter arrived within five minutes.
Mr Draper, of the Born Free Foundation, said the charity is now calling on the Zoos Expert Committee - the government advisory committee on zoo matters- to investigate the safety and welfare of great apes in UK zoos, and for the Zoo Licensing Act and associated standards to be amended to improve the system of zoo inspections, as well as increase the penalties for zoos found to be putting their animals or visitors at risk.
A spokesman for London Zoo said they were unaware of any previous problems with the glass enclosure and confirmed the incident was being fully investigated.
According to the zoo's website, there are at least six gorillas living in its Gorilla Kingdom.
Among them is Kumbuka, a western lowland silverback, who arrived at ZSL London Zoo in early 2013 from Paignton Zoo in Devon.
Others include Zaire, who came to London Zoo in 1984 after being born in Jersey Zoo, Mjukuu and her daughter Alika, 'teenager' Effie, and Gernot, the latest addition who was born in November last year to Effie and Kumbuka.
When 7ft tall Kumbuka arrived at the zoo, in the hope he could boost the numbers of the critically endangered species in the European breeding programme, keeper Daniel Simmonds said: 'We've been slowly introducing him to our three females, and Mjukuu and Effie are already quite smitten with their handsome new roommate.
'He is proving to have quite a playful side to him; he loves to snap the branches on the smaller trees on their island and standing in the spray of the hosepipe when we clean his dens.'
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'.The lowland gorilla is an endangered species.
Kumbuka was previously kept at Paignton Zoo where he was photographed by wildlife photographer Richard Austin.
Speaking at the time he said: 'Of all the animals I've encountered here in the Westcountry, Kumbuka the silverback lowland gorilla at Paignton Zoo is one of my favourites, despite the fact that every time I photograph him he tries to take my head off with missiles made of mud, turf and even a hard lump of flint the size of a house brick.
'For some reason unbeknown to me he doesn't like cameras or photographers, or maybe just press photographers.'
Another woman, who asked not to be named, told MailOnline: 'This gorilla did actually the same thing to myself and my son who was two at the time, when he was in Paignton Zoo, Devon.
'We were watching him and from nowhere, he jumped and threw himself at the glass.
'It was a very short scary experience for us and who even now my son gets nervous when visiting the gorillas.
'The zoo should definitely put up a sign or remove Kumbuka from public viewing, he is a beautiful gorilla and I certainly wouldn't want him destroyed but he is clearly a danger to himself and the public.'
Kumbuka 'should not be put back on show' says leading primate expert
A leading primate expert said Kumbuka should not be put back on show to the public at London Zoo.
Professor Phyllis Lee said staff at ZSL London Zoo should consider moving the gorilla to another zoo elsewhere in Europe because he is 'unhappy in his environment.'
Professor Lee is an expert on primate psychology, based at Stirling University.
She told the Evening Standard: 'He would have been somewhat challenged in his environment to start with, and from the sound of his behaviour he was desperate to escape.
'They are highly intelligent and if there is a way to escape they will find it.
'He was obviously clearly unhappy in his environment and wanted to escape, to find a new group and females to mate with. He would not have been happy with the other gorillas at the zoo.
'I now expect him to be transferred.'