There is no evidence of shots being fired or any casualties on London's busiest shopping street, police said, after responding to reports of gunfire.
There was a "significant level of panic" on Oxford Street and at Oxford Circus Tube station as witnesses said they heard shots.
But after armed police responded to the area which was packed with Black Friday shoppers and rush-hour commuters, they announced nothing had been found and the Tube station has re-opened. One woman suffered minor injuries in the crush as the station was evacuated.
At 6.06pm the Metropolitan Police said its response had been "stood down".
"If you sought shelter in a building please now leave, and follow the direction of police officers on the ground if you need assistance."
Additional officers remain on duty in the West End to reassure the public. We thank the public for their patience and assistance during our response. If you see anything suspicious dial 999 immediately #OxfordStreet
— Metropolitan Police (@metpoliceuk) November 24, 2017
In a statement British Transport Police said: "At 4.37pm this afternoon, officers were called to Oxford Circus station following reports of gunfire on the westbound Central Line platform.
"Passengers at the station then self-evacuated the station onto Oxford Circus and Regent Street area of London.
"This caused a significant level of panic which resulted in numerous calls from members of the public reporting gunfire.
"Officers responded in line with our procedures of a terrorist incident; this included armed officers from British Transport Police and the Metropolitan Police. A full and methodical search of the station and Oxford Street was conducted by officers.
"At this stage, we are examining the circumstances of the incident which resulted in the station being evacuated.
"During the station evacuation, one woman is believed to have sustained a minor injury."
There were reports of people running out of the Tube station and police urged people to take cover inside buildings.
In a statement the Metropolitan Police said it was alerted at 4.38pm "to a number of reports of shots fired on Oxford Street and underground at Oxford Circus tube station".
"Police have responded as if the incident is terrorist related. Armed and unarmed officers are on scene and dealing along with colleagues from British Transport Police.
"If you are on Oxford Street go into a building and stay inside until further direction.
"Avoid travelling to the Oxford Street area. At this stage police have not located any casualities."
At 5.42pm the Met Police tweeted:
We have not located any trace of suspects, evidence of shots fired or casualties. Officers still on scene. If you are in a building stay there, if you are on the street in #OxfordStreet leave the area. Officers continue to search the area. More updates as soon as we have them
— Metropolitan Police (@metpoliceuk) November 24, 2017
Transport for London had tweeted: "Oxford Circus Station is closed while we investigate a customer incident."
London Fire Brigade had three fire engines and 15 fire fighters at the scene.
People were seen running away from Oxford Circus station in tears as the alert unfolded.
A huge cordon was put in place straddling some of the English capital's busiest shopping streets, which was being expanded as the situation developed, while shops on Oxford Street and Regent Street were also being evacuated.
Police set up one of the cordons across Oxford Street at Holles Street by the John Lewis department store.
Dozens of uniformed officers came out in lines and told people to get off Oxford Circus and knocked on the doors of all the shops along the road, saying the area needed to be evacuated.
At 5.30pm, just weeks before Christmas, the busy shopping destination had been packed with people spending money.
Buses were stopped and were lined along the side of the road.
The London Palladium on Argyll Street, which is due to host the Royal Variety Performance later, was in lock-down.
Lanna Woodward (20) from California, who is visiting London said she did not see any shooting but witnessed the aftermath of panic.
"Before it all I saw about 30 police officers speeding down the street," she said.
"We were in the Kingdom of Sweets and then saw a massive crowd of people running and screaming.
"People were climbing on top of each other.
"The store we were in was locked down, the manager wouldn't let us leave - they said there had been a shooting at the entrance of the Tube station."
Bond Street Station closed to prevent overcrowding. All trains not stopping and paper tickets will be accepted on local buses
— TfL Travel Alerts (@TfLTravelAlerts) November 24, 2017