This is the terrifying moment an angry 29-stone gorilla escaped from an enclosure at London Zoo after crowds were warned about taunting him – as it was revealed the glass casing may have been cracked before the primate escaped.

Eighteen-year-old male silverback Kumbuka charged at the glass and managed to successfully break it before escaping the enclosure and roaming the north London attraction for a horrifying 90 minutes.

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.

One zookeeper allegedly told BuzzFeed news: 'He's a f*****g psycho, that ape. He's smacked the enclosure glass a couple of times.'

This is the moment Kumbuka attacked the glass in London Zoo with 50 people watching

Witnesses said the gorilla launched himself at the glass as some visitors tried to wind him up

Approximately 100 visitors were locked up while police and zookeepers searched for the ape

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.

A spokesman for London Zoo this morning said Kumbuka was treated by keepers overnight who stayed to observe him. The zoo said he has returned to the main enclosure and an investigation was 'ongoing'.

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 warned of damage to the perimeter glass of the enclosure in the weeks prior to the incident.

Annie Bristow made a video of Kumbuka thrashing a rope around in an attempt to smash the glass, in September 2015. She added: 'So this has been going on for a while'

There was a police presence outside London Zoo this morning following 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.

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 Witness Chloe Hughes

'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.'

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.'

One eyewitness, Oliver Barker, 24, said a zoo keeper warned visitors against 'staring' at one of the male gorillas, believed to be Kumbuka, pictured, as this would 'aggravate him'

Heavily armed police raced to the zoo after they received reports of the escaped gorilla

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.'

He looked like he was in a sort of trance, he definitely wasn't relaxed at all. He looked very intimidating Zoo visitor Oliver Barker

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.'

Veterinary student Chloe Hughes, centre, witnessed Kumbuka ram the enclosure's glass. Ms Hughes was with her brother Alex, left, and Amelia, right for a day trip to London Zoo

Veterinary student Chloe Hughes, pictured here at the zoo before the escape, recorded the very moment Kumbuka charged the glass in his enclosure in his effort to get out

Mr Barker posted a photograph of Kumbuka shortly before this afternoon's dramatic escape

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.'

Officers armed with Heckler & Koch MP5s patrolled the zoo in an effort to find Kumbuka

It appeared like the armed officers were tasked to protect zoo keepers looking for the gorilla

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.'

London Zoo said Kumbuka was now back in his enclosure 'enjoying his favourite treats'

The zoo was on lockdown after the massive gorilla burst out of his enclosure

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.'

Armed zoo keepers hunt the gorilla as members of the public are locked inside the cafe where this picture was taken

Visitors to the Zoo report that helicopters are flying overhead and they can hear sirens

Staff in the zoo handed out free teas and coffees to visitors while armed police patrolled

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.'

A spokeswoman for London Zoo confirmed they 'managed an incident on site'

The gorillas at London Zoo live in a purpose-built enclosure which opened in 2007

Rugby legend Matt Hampson was at London Zoo this evening when the gorilla escaped

Hannah O'Donoghue-Hobbs and her friend Charlotte were locked in during the incident

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.

The gorilla managed to escape from the Gorilla Kingdom enclosure which opened in 2007

Staff warned visitors to leave the gorilla enclosure when Kumbuka started bashing the glass

Members of the public were locked into a butterfly enclosure while staff hunted the gorilla

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 seven 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.

Police launched a major security alert after the gorilla escaped from its enclosure

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.'

Armed police patrolled the zoo for 90 minutes while they attempted to recapture the ape

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.

London Zoo insist that all members of the public were safe despite the escaped gorilla

Visitors to the zoo said they witnessed zoo keepers with tranquillizer guns looking for the ape