Emergency services were called shortly after 10.30pm following reports that a person had fallen from a bridge onto the railway tracks near Hove railway station.
Emergency services were mobilised to the area, with railway power switched off to allow crews to safely deal with the incident.
Emergency services remain at the scene, with the circumstances of the incident not yet known.
Railway lines closed between Brighton and Worthing
Southern has confirmed that all railway lines between Brighton and Worthing are currently closed while emergency services deal with the incident.
The rail operator said trains may be delayed by up to 60 minutes or revised, with disruption expected to continue until the end of the day.
Southern said services are currently unable to run between Brighton and Worthing, including services operating via Preston Park.
The operator is currently unable to provide an estimate for when the railway lines will reopen.
Customer advice
Southern says tickets will be accepted on alternative Southern and Thameslink services at no additional cost.
Passengers are being advised to listen to station announcements, check station screens and consult journey planners for the latest information.
Replacement buses are also running through the affected area as part of pre-planned engineering works, operating between Brighton, Worthing and Littlehampton, with stops at stations along the route.
Passengers can use the replacement buses to complete their journeys.
This is a developing story. Further updates will be added as more information becomes available.
