Police scrambled to Hertford Road, in Enfield, to reports of a firearm incident at around 4.40pm on Thursday.
Officers and paramedics found a man with a head injury at the scene, Scotland Yard said.
A spokeswoman said: “He has been taken to hospital where he is in a life-threatening condition.”
No arrests have been made and police are launching an investigation into the incident.
A cordon remains in place while police investigate.