ZSL London Zoo said it was "managing an incident" following reports of an escaped gorilla.
The gorilla is thought to have escaped from its enclosure at the central London zoo, with reports that the public have been locked in.
Footage posted on social media from the zoo shows armed police walking through the zoo's grounds.
-- hannah (@hannahod) October 13, 2016
A zoo spokesman said: "ZSL is managing an incident on ZSL London Zoo."
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."
Stuck inside Butterfly Paradise at London Zoo as helicopters and sirens all around, something happening with gorillas. #Weird
-- Cris (@crisclark) October 13, 2016
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.