Officers were called to the incident in Whitehawk Road, Whitehawk, Brighton, at 2.30pm today (April 5).
A Sussex Police spokesman said: “Detectives are investigating a serious incident in a street in Whitehawk.
“The victim, a man believed to be in his 20s, was taken to Royal Sussex County Hospital in Brighton with serious injuries.
“A man was arrested at the scene in connection with the incident.
“A section of the road is currently closed and local residents can expect to see a significant police presence in the area as enquiries continue.”