London Zoo was put on lockdown after a gorilla escaped from its enclosure.
Armed police were called to the central London attraction and visitors were evacuated as keepers desperately searched for the loose animal.
Others were locked inside buildings at the zoo, one of London's top tourist spots, which is in the grounds of Regent's Park.
Scotland Yard said the incident was "concluded" at around 6.45pm amid reports the gorilla had been shot with a tranquilliser dart and recaptured.
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."
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, which is an endangered species. He 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.
It is not yet clear how the primate got out.
Visitors to the gorilla attraction reported seeing the "lead male" apparently agitated and charging at the enclosure's glass walls moments before the escape.
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."
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 12ft into the exhibit at Cincinnati Zoo.
Zoo officials made the decision because they felt the boy was in a "life-threatening situation".