According to traffic reports, Brighton Road is closed in both directions due to the accident, which reportedly happened between the junction with the B2223 and Seamill Park Avenue.
It is alleged to have been a collision involving a motorbike and a car.
Traffic is being diverted via roads nearby, but it is causing gridlock across the town. The A259 and the A27 is queueing westbound back to Shoreham, with 'severe delays', according to traffic reports.
This comes as the A280 is closed between Angmering and Worthing after a motorcyclist was seriously injured.
A Sussex Police spokesman said a number of vehicles were at the scene but did not have more information about the collision at the time of writing.