Eyewitness Brad Evans, who has been 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."
Scotland Yard said an incident involving an escaped gorilla at London Zoo has "concluded", amid reports it has been shot with a tranquilliser dart and recaptured.
It is understood staff are armed with tranquilliser darts.
Members of the public were ordered to retreat to safety and were in lockdown in sites across the zoo.
The gorilla was recaptured after more than three hours on the run.
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!"
A spokeswoman from ZSL London Zoo simply told the Mail Online: "We managed an incident on site".
She declined to confirm or deny that the animal had smashed through its glass enclosure, or that it was silverback gorilla Kumbuka who had escaped.
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.
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 seven-foot-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."
The incident comes just months after staff at Cincinatti Zoo shot dead Harambe after he grabbed a child.
