An "altercation" between two men on a Tube platform is thought to be the reason behind a mass evacuation in central London on Friday, police have said.
Several people were injured and nine were taken to hospital after chaos and confusion erupted at the busy Oxford Circus station.
Armed police swooped on the station and along Oxford Street after receiving reports that shots had been fired.
A British Transport Police (BTP) spokesman said: "Armed officers from BTP and MPS were quickly on scene. The area was searched swiftly and the station was re-opened.
"Officers believe an altercation erupted between two men on the platform."
They issued CCTV images of two men they want to speak to and have appealed to anyone with information to come forward.
Chief Superintendent Martin Fry said: "Thank you to everyone for bearing with us this evening and also to colleagues from all the emergency services who helped carry out a swift response.
"I know incidents like these can cause concern, but our officers are highly visible around the network and across the country."
Stuart Crichton, London Ambulance Service assistant director of operations, said paramedics treated several patients who sustained injuries while leaving Oxford Circus.
He said: "We discharged seven patients at the scene and took eight patients to two central London hospitals for minor injuries.
"We also took one patient to a major trauma centre for leg injuries."
Shortly after 4.30pm on Friday police were called to reports of gun shots. Crowds of people poured out of Selfridges department store, all Underground trains in the area were halted, traffic was blocked and officers locked people inside shops.
It was an hour-and-a-half before Met Police gave the all-clear.
Eyewitnesses reported being caught in "stampedes" and said there was "pandemonium" as police treated the incident as if it was terrorist-related.
However, Met Police said they could find no evidence of shots fired.
They added: "Given the nature of the information received, the Met responded in line with our existing operation as if the incident was terrorism, including the deployment of armed officers.
"Officers working with colleagues from British Transport Police carried out an urgent search of the area."
British Transport Police said there was "a significant level of panic", adding that it was "examining the circumstances of the incident which resulted in the station being evacuated".
Pictures and video on social media showed armed officers at Oxford Circus station and people being led away from Oxford Street.
Selfridges later said the evacuation of its shop was a precautionary measure and there were "no reported incidents in store".
Bryce Malcolmson, who works in Soho, said he was in Oxford Circus Tube station when he heard a call over the loudspeaker for a police officer to attend one of the platforms.
He told Sky News: "Suddenly I just heard screaming and people running out of the station. Everybody turned round in sort of pandemonium and started trying to exit the Tube as quickly as possible.
"I got up to the top by the ticket barriers and they were all open. I ran out on to the street but it was almost as if no one knew anything had happened down below.
"Within… a couple of minutes there were armed response units and police cars absolutely everywhere. I think people were so unsure what had happened they were running in all directions."
Tourist Lanna Woodward said she witnessed the aftermath of the panic on Oxford Street.
The 20-year-old from California said: "Before it all I saw about 30 police officers speeding down the street.
"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."
Paying tribute to the "swift response" of emergency services, mayor Sadiq Khan urged Londoners to "remain vigilant and don't panic".
He said: "It is vital that we are not complacent – if in doubt it is always best to err on the side of caution and call the police on 999."
:: Anyone with any information can call BTP on 0800 40 50 40 or text 61016 citing reference 405 of 24/11.