Work is taking place between Haywards Heath and Lewes, closing all lines.
Buses will replace trains between Haywards Heath and Lewes via Cooksbridge and Plumpton, Southern said.
A revised service will operate between Brighton and Eastbourne/Bexhill.
Passengers travelling between Bexhill, Eastbourne, Lewes and London Victoria should travel via Brighton, Southern said.
Southeastern services between London Cannon Street, London Charing Cross, London Bridge and Hastings and Southern services between Ore, Brighton and Hastings and Ore and Ashford International are all closed as engineering work is taking place in the Hastings area.
Buses will replace trains between Bexhill and Ashford International, as well as trains between Battle and Hastings.
Due to limited road access, these buses will not stop at Crowhurst.
Passengers should change at Battle for a minibus service to and from Crowhurst.
Elsewhere, the roads across the county are reported to be free of congestion as of this morning (Saturday).
A message from the Editor, Gary Shipton:
In order for us to continue to provide high quality and trusted local news, I am asking you to please purchase a copy of our newspapers.
With the coronavirus lockdown having a major impact on many of our local valued advertisers - and consequently the advertising that we receive - we are more reliant than ever on you helping us to provide you with news and information by buying a copy of our newspapers.
Our journalists are highly trained and our content is independently regulated by IPSO to some of the most rigorous standards in the world. But being your eyes and ears comes at a price. So we need your support more than ever to buy our newspapers during this crisis.
Stay safe, and best wishes.