CCTV showed passengers on the train watching the fight
An off-duty police officer was seriously attacked on a train in West Sussex after trying to stop a group of youths verbally abusing passengers.
British Transport Police (BTP) said the fight was on a Portslade-Lancing train.
Pc Jason Scrase said youths who boarded the train with full pint glasses turned on the officer when he tried to stop their anti-social behaviour on Friday.
Four people were later arrested on suspicion of causing actual bodily harm and assaulting a police officer.
'Passengers saw fight'
Pc Scrase said: "CCTV of the incident shows members of the group were carrying full pint glasses and they were being anti-social towards several passengers on the train.
"An off-duty police officer intervened to calm the situation, however the group then turned on the officer and he was subsequently assaulted."
CCTV also showed passengers watching the fight, which left the officer with cuts and bruises, from the rear of the carriage.
The group left the train at Lancing, Pc Scrase said.
He said a 16-year-old, two 17-year-olds and a man aged 26, were arrested a short time later, and bailed until 17 March.
The train was "quite busy" at the time, but police had so far only spoken to a handful of witnesses, he added.
Urging people who witnessed the incident to come forward, Pc Scrase said: "The slightest bit of information can be pivotal to the investigation."