A family was forced to flee their home after a man barricaded himself inside with a gas canister, sparking a large police response.
Officers and emergency services have cordoned off Daneby Road, Catford since around 6.30pm on Monday night.
According to reports, the suspect has a 15kg gas cylinder with him.
The other occupants managed to escape the semi-detached house worth around £700,000.
Twenty-two residents evacuated from their homes are being supported by Catford Salvation Army, who described the incident as a “dangerous situation”.
Specialist firearms officers who deal with counter-terrorism incidents were deployed to the scene, along with uniformed teams from Bromley, Catford and Lewisham.
The Metropolitan Police advised people to avoid the area while negotiators speak to the man by phone.
Several London Fire Brigade engines and paramedic crews are on standby amid a substantial cordon. Building Blocks Day Care nursery on the street had to close on Tuesday.
In a Facebook post, the Salvation Army said: “We are still assisting with the response to the dangerous situation in Catford.
“We are hosting 22 local residents while they have been evacuated from their homes. Many of whom have not slept all night.
“Please pray for them and the man they are trying to get to surrender.”
A police spokesman said: “Officers and emergency services are on scene Daneby Road, SE6 after concerns were raised for the welfare of a man inside a property at the location.
“No one else is believed to be inside.
“Officers are attempting to engage with the man and bring the matter to a safe conclusion.
“Cordons and road closures remain in place while the situation is ongoing.
“We thank local residents for their patience and advise people to avoid the area.”
Bus services leaving the nearby Catford Road bus garage have been rerouted.
One journalist at the scene claimed police decided to declare a major incident following the discovery of additional gas cylinders on the property, indicating a potential threat to public safety. Scotland Yard did not comment.
In a separate incident last December, a 26-year-old man was arrested after a “quantity of chemicals” was found at a suspected bomb factory in Daneby Road.
LFB and London Ambulance Service said the incident is being led by the Met.
Lewisham Council were approached for comment.
