Police were called at 14:02 on Monday, 22 June to Weir Road, SW12 to reports of a man with a firearm.
Officers attended, along with LAS as a precaution, and a man, no further details, has been arrested on suspicion of possession of a firearm with intent. He has been taken to a London police station, where he remains.
Road closures are in place in the surrounding area.
Officers remain on scene and enquiries continue. There have been no reports of any injuries.