Police, ambulance and the fire service are at the scene in Bromley, south London.
According to police, a firefighter has injured his hand due to the substance, which has not yet been identified.
Some residents at the block of flats, in Homesdale Road, have reportedly been injured, but not seriously.
In a statement, Bromley MPS said: “Police were called at 14:46hrs on Saturday, 27 July following reports of a suspected corrosive substance thrown over a communal doorway inside a residential building in Homesdale Road, Bromley.
“Officers are in attendance with LAS (London Ambulance Service) and LFB (London Fire Brigade).
“Some residents are reported to be suffering from irritation as a result.
“No one is thought to be seriously injured.
“A firefighter is believed to have sustained an injury to his hand from the substance; enquiries continue to determine what the substance is.”
In an update at about 4.30pm, police said officers were still at the scene.
Anyone who has information that could assist police with regards this incident should call 101 or tweet @MetCC ref CAD 4684/27 July.
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