A photo has emerged showing a man hanging out of a window wielding a knife in Lewisham, south-east London.
The man can be seen in a grey dressing gown, brandishing a blade in flats opposite banks HSBC and Barclays.
Armed police are currently on Lewisham High Street, along with fire crews and paramedics.
Reports on social media have suggested a hostage situation was underway but this remains unconfirmed.
The Metropolitan Police said officers are working to find out whether anyone else is in the flat.
A spokesman for the force said: “Police were called to a disturbance on Lewisham High Street, at around 10.45am on Monday.
“Officers are currently in attendance at the scene alongside the London Ambulance Service and the London Fire Brigade.
“We are speaking to a man inside an address at the location.
“Inquiries are ongoing to establish whether anyone else is inside the property.”
The High Street remains closed while the police, ambulance and fire services are at the scene.
An HSBC spokesman said it has closed its Lewisham branch “as a precuation”.
The spokesman said: “To clarify, the incident is not taking place inside the HSBC branch and no staff or customers are involved, however we have closed the branch as a precaution.
“This is a police matter, and we remain in communication with the local authorities.”
The London Fire Brigade said its crews remain on the scene after being called at 10:56.
Robert Campbell, a Greenwich resident who came to Lewisham to use the gym, said: “They need to go in there. Lacking as well is any kind of negotiation. There should be someone trying to reason with the guy.
“They need to step up. Escalate the situation. It’s got to come to a close at some point. I mean they’re all just standing around.”
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