Armed police have evacuated Oxford Circus station amid reports of smoke.
Ryan Butcher, an Independent reporter who was inside the station at the time, said panic broke out after passengers were told to evacuate the station.
He said a tannoy announcement said there had been a “reported emergency” and police arrived quickly.
"Everyone started running," he added. "I saw smoke...people started falling over in the streets."
Mr Butcher is sheltering in a nearby restaurant, where he said other people were "shaking and crying", adding: "It's just terrifying."
Police are urging everyone in the Oxford Street area to go into a building. Armed officers are on the scene.
British Transport Police did not immediately confirm the cause of the alert, which came as shoppers flocked to central London on Black Friday.
"We continue to respond to an incident at Oxford Circus," a spokesperson said. "The station is currently closed, please avoid the area at this time."
The Metropolitan Police said its officers were supporting BTP but could give no further information on the incident.
Trains are not stopping at Oxford Circus and tickets are being accepted on local buses.
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