Southern Rail reported on Twitter, at 3.45pm, that delays were expected but advised people to 'travel as normal'.
"As a result of an earlier incident of passengers causing a disturbance on a train between Ford and Barnham, delays are expected," a Southern spokesperson said.
"You can travel as normal, but we'd advise leaving an extra 15 minutes if you're planning to travel through the area."
Passengers reported on social media that the affected train, which was heading towards Southampton Central, was diverted to Bognor Regis.
In an update at 4.20pm, Southern said there were still some delays on a handful of services along the coast towards Bognor Regis, Portsmouth and Southampton.
Services have since returned to normal.
If you've been delayed by 15 minutes or more this afternoon, you are advised to claim delay repay.