Police were called to the assault on Upper Elmers End Road just before 8.30pm yesterday (March 16).
Officers and the ambulance service found a man with head injuries and took him to hospital in a life-threatening condition.
A 29-year-old man was arrested on suspicion of grievous bodily harm and is in custody.
Nobody else is sought by the police but their enquiries are ongoing.
A Metropolitan Police spokesperson said: “On Sunday, 16 March police were called to Upper Elmers End Road, Bromley at 20:24hrs following reports of an assault.
“Police attended the scene alongside the London Ambulance Service and treated a man in his 40s for head injuries before taking him to a nearby hospital.
“At this time, the man’s injuries are considered to be life-threatening.
“A 29-year-old man was arrested on suspicion of GBH, he remains in police custody.
“Police are not seeking anyone else in connection to this incident.
“Enquiries are ongoing.”
A London Ambulance Service spokesperson said: “We were called at 8.21pm on Sunday 16 March to reports of an assault on Upper Elmers End Road, Beckenham.
“We sent resources to the scene, including an ambulance crew, an advanced paramedic and an incident response officer.
“We also dispatched a trauma team in a car from London’s Air Ambulance.
“We treated a man at the scene and took him to a major trauma centre by road.”
