Cops and emergency services scrambled to the scene on Eden Street in South London just after 8am this morning.
Officers had been alerted to reports of a man holding a hammer and acting aggressively in Kingston.
After arriving, the 21-year-old man was found with a head injury and rushed to hospital.
Another man was arrested nearby on suspicion of causing grievous bodily harm, and remains in police custody.
A post shared on Twitter this afternoon shows a police cordon in place on Eden Street.
A spokesperson for the Met Police said: “Police were called at about 08.10 on Saturday, September 2 to reports of a man in possession of hammer, and acting aggressively on Eden Street in Kingston.
“Officers attended and found a 21-year-old man with a head injury.
“He was taken to hospital by the London Ambulance Service; we await an update on his condition.
“A man was detained nearby and subsequently arrested on suspicion of causing grievous bodily harm. He remains in police custody.”