Armed police are responding to reports of shots being fired at London's Oxford Circus Tube station.
The Met said it was responding as if the incident was terrorist related and urged people on nearby Oxford Street to take cover inside buildings.
"Oxford Circus Station is closed while we investigate a customer incident," Transport for London said.
BBC reporter Helen Bushby said she had seen a "mass stampede" of people running away from the station.
The Met said armed and unarmed police were at the scene.
"At this stage police have not located any casualties," the force said in a statement.
In a statement on Twitter, British Transport Police said: "We continue to respond to an incident at Oxford Circus.
"The station is currently closed, please avoid the area at this time. Officers are on scene."
The BBC's Helen Bushby said she had seen people "screaming and crying".
She spoke to a group of young women at Topshop, on Oxford Street, who said people had dropped their shopping and ran as quickly as they could.