A gorilla was shot with a tranquilizer dart after escaping from its enclosure at London Zoo.
Others were locked inside buildings at the zoo, one of London's top tourist spots, which is in the grounds of Regent's Park.
Armed police were called to the central London attraction and visitors were evacuated as keepers desperately searched for the western lowland silverback named Kumbuka.
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."
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."
Neuropsychologist Dr Jonathan Mall was at a conference at the zoo and had taken a break. He 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 tried to bring the loose gorilla 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**t out of me."
Eventually staff went from building to building to rescue visitors and evacuate them from the zoo.
According to the zoo's website there are at least seven gorillas living in its Gorilla Kingdom.
Kumbuka arrived at ZSL London Zoo in early 2013 from Paignton Zoo in Devon.
Scotland Yard said the incident was "concluded" at around 6.45pm amid reports the gorilla had been shot with a tranquilliser dart and recaptured.