Police closed Portland Road in Hove after a man was attacked in the street in what has been described as a serious assault.
The incident appears to have been a case of road rage after a car driver is believed to have opened his door and hit or nearly hit a cyclist.
One of two or three cyclists riding together is thought to have attacked the driver who was taken by ambulance to the Royal Sussex County Hospital in Brighton for treatment.
Detective Sergeant Julian Deans, of Sussex Police, urged the attacker and his fellow cyclist or cyclists, who were involved in the incident just before midday, to contact him.
A bicycle was abandoned at the scene.
Police closed the road shortly at about noon although traffic was allowed to start heading westbound from between 1pm and 2pm.
Buses and other traffic are being diverted.