London's Oxford Circus underground station in the heart of the British capital's main shopping district was being evacuated on Friday evening and armed police were on the scene, a Reuters witness said.
"If you are on Oxford Street go into a building. Officers are on scene and dealing (with it)," the Metropolitan Police said on their official Twitter feed.
British Transport Police said officers were on the scene responding to an incident but gave no further details.
"Officers are responding to reports of an incident at Oxford Circus station," British Transport Police said on Twitter. "The station is currently closed, please avoid the area at this time," they said.
Transport for London said: "Oxford Circus Station is closed while we investigate a customer incident."
Witnesses on social media reported people running into nearby shops and pubs.
Two London underground stations on Oxford Street reopened later after the police initially treated the incident as if it might be terrorist-related, the city's transport authority said on Friday.
"Oxford Circus and Bond Street stations now both reopened and all trains are stopping normally," the transport authority said on one of its official Twitter accounts.