Officers want to speak to anyone who was at Oxford Circus underground station at the time of the evacuation.
Sixteen people were treated after they were injured fleeing the station, following reports of gunshots being fired on a Central Line platform.
There was no evidence any weapons had been fired, police said.
Shoppers were barricaded inside stores on Oxford Street and armed police were deployed after the alarm was raised during the evening rush hour on Black Friday.
Police initially treated the incident as potentially terrorism-related, before standing down.
The Metropolitan Police said it began receiving "numerous" 999 calls reporting gunshots in Oxford Street and at Oxford Circus station at 16:38 GMT on Friday.
The first armed response vehicle was on the scene in less than a minute from receiving the first call, the force said.
Oxford Circus - where Oxford Street and Regent Street meet - was cordoned off, while shops and businesses were placed in lockdown.
In a statement, the Met Police said: "No casualties, evidence of any shots fired or any suspects were located by police."
But 16 people were injured as passengers fled from Oxford Circus station, in what witnesses said was "a stampede".
One patient was transferred to a major trauma centre for leg injuries, while eight people were taken to central London hospitals for minor injuries.
By 18:05 GMT, the police operation had been stood down.
In a statement, Mayor of London Sadiq Khan praised the city's emergency services for a "swift response".
'Mass stampede'
"People said they heard a gunshot and panic was just spreading."
correspondent Andy Moore, eyewitnesses reported being evacuated from the tube station on the instructions of a generic announcement.
They came up to street level in a state of confusion - some reporting loud bangs - spreading panic among shoppers which quickly passed down the street and onto social media.
The British Transport Police told those in the area to "go into a building and stay inside until further notice".
People barricaded themselves into shops, many of which went into lockdown - including Selfridges, which is half a mile down Oxford Street from Oxford Circus.
Singer Olly Murs, who was shopping in Selfridges, told his 7.8m followers on Twitter: "Get out of Selfridges now gun shots!! I'm inside."
He added: "Really not sure what's happened! I'm in the back office... but people screaming and running towards exits!"
Mr Murs, who was later criticised for spreading panic, said afterwards: "It's easy to say now it was nothing but in a state of shock and panic I was trying to make people aware of what was happening. Which I was led to believe by staff and customers that someone was shooting."
Selfridges later said on Twitter that it was evacuated "as a precautionary measure".
It added: "We have been working with @MetPoliceUK and can confirm that there were no reported incidents in store."
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