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Armed police were called to London Zoo this evening after a silverback gorilla "charged at its enclosure window" and escaped from its den.

In a terrifying turn of events "loose animal" sirens blared out and the zoo was on lock-down, with scared visitors trapped in buildings as staff attempted to sedate the animal.

Eyewitnesses reported seeing armed police and helicopters before keepers sedated and recaptured the gorilla.

It is not clear how the aminal got out of its enclosure but zoo visitors said the the silverback was agitated and charging at the glass seconds before the escape.

The huge animal was seen repeatedly banging his 29-stone body into the glass repeatedly just before alarms started ringing and visistors were ushered to safety.

Jonny Briers, 22, from Tooting told The 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.

“We were in there for about half an hour to 45 minutes. We didn’t know what was happening. A zookeeper kept us calm and said it was an animal escape siren.

“People were let out at around quarter to six.

“The doors to the aquarium were automatic so we didn’t feel that safe, they were glass doors.

“Some people were pretty scared. We did see a woman on a bench hyperventilating as we were leaving.”

The head of mammals at the zoo dismissed the escape as a "minor incident" and a zoo spokeswoman added the gorilla was now "awake and well".

She said: "A male gorilla got out of his den at ZSL London Zoo and into a non-public keeper area at 5:13pm on Thursday, 13 October 2016. The gorilla remained contained within the exhibit’s off show area.

"Staff responded immediately and the 18-year-old gorilla - named Kumbuka - was tranquilized by vets and returned to his den, where we can confirm he is awake and well.

"It is not yet confirmed how the incident occurred and a full investigation will be carried out to determine what happened.

"The exhibit is secure and we are grateful to all of our staff and visitors for their cooperation, enabling us to resolve the situation quickly and efficiently."

The escape comes after, in May, a gorilla was shot dead by keepers when 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.

Metropolitan police were called to the zoo at 5:20pm “to support zoo staff dealing with the gorilla in question."

A spokesman confirmed at 6.48pm that the incident had been "concluded".

Rob Hogan, 37, told the Standard the gorilla seemed “calm and peaceful” just minutes before it charged at the glass right in front of his eyes.

Mr Hogan, who had come to London Zoo from Halifax for a work conference, said: “A small group of us went to the gorilla enclosure and started to take pictures, no flash.

“I took a picture and then one second, two seconds later, he jumped at the window. It sounded like a car hitting the window.

“We stayed there for another two or three minutes looking, then we walked off towards the reptile enclosure. Two minutes later we heard an alarm.

“We saw park wardens moving to the gorillas. They said it was a drill. But we put two and two together, because it just jumped at the window. You don’t do that for a meerkat.”

Eyewitness Brad Evans, who was allowed to leave the zoo, 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."

He arrived in early 2013 from Paignton Zoo in Devon and is the father of the two infants Alika, 3, and Jernot who was born last year.

According to the zoo's website there are at least seven gorillas living in its Gorilla Kingdom.

Eighteen-year-old Kumbuka is the only male silverback and is the zoo's star attraction.

He arrived in early 2013 from Paignton Zoo in Devon and is the father of the two infants Alika, 3, and Jernot who was born last year.

At an impressive 29 stone Kumbuka is thought to be over six times stronger than the average man.

Videos of lowland gorillas show the animal snapping trees in half with incredible strength.

As the male, Kumbuka is the leader of the “troop” at the zoo.

Kumbuka the gorilla 5 show all Kumbuka the gorilla 1/5 AN20268351Newly arrived at .jpg New arrival: Kumbuka meets the public at London Zoo. He arrived two weeks ago from Paignton Zoo in Devon (Picture: Glenn Copus) Glenn Copus 2/5 AN20268395Newly arrived at .jpg Ladies' man: despite never having had a female mate before, Kumbuku has already been spotted flirting with females Mjukuu and Effie Glenn Copus 3/5 kumbuka-jpg.jpg At home: Kumbuka with a green pepper as he settles in at London Zoo Getty 4/5 AN20268475Newly arrived at .jpg Majestic: Kumbuka is a 15-year-old western lowland silverback Glenn Copus 5/5 kumbuka.jpg Powerful: the western lowland gorilla Getty 1/5 AN20268351Newly arrived at .jpg New arrival: Kumbuka meets the public at London Zoo. He arrived two weeks ago from Paignton Zoo in Devon (Picture: Glenn Copus) Glenn Copus 2/5 AN20268395Newly arrived at .jpg Ladies' man: despite never having had a female mate before, Kumbuku has already been spotted flirting with females Mjukuu and Effie Glenn Copus 3/5 kumbuka-jpg.jpg At home: Kumbuka with a green pepper as he settles in at London Zoo Getty 4/5 AN20268475Newly arrived at .jpg Majestic: Kumbuka is a 15-year-old western lowland silverback Glenn Copus 5/5 kumbuka.jpg Powerful: the western lowland gorilla Getty

According to the National Geographic, the alpha male defends his troop from threats with “impressive shows of physical power”.

He is likely to “stand upright, throw things, make aggressive charges, and pound his huge chest while barking out powerful hoots or unleashing a frightening roar”.

The animal is an endangered species and can reportedly experience stress when held in captivity.