The gorilla is thought to have escaped from its enclosure at the central London zoo, with reports that the public have been locked in.
A zoo spokesman said: “ZSL is managing an incident on ZSL London Zoo.”
Footage posted on social media from the zoo shows armed police walking through the zoo’s grounds.
Scotland Yard said: “Police were called by security at Regent’s Park at 5.17pm to reports of an ongoing incident.
“Officers are there to support ZSL staff.”
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.”
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.
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.