A firefighter was taken to hospital with head injuries after he was attacked by moped riders in Tesco Wick car park.
The attack happened on Monday night, police say, when the firefighter was in full uniform.
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The 62-year-old local man was taken to Worthing Hospital.
According to police, the firefighter was reported to have had a verbal altercation with three moped riders.
He was then struck with a helmet and a tool, causing him to fall to the ground.
The suspects then made off from the scene, and members of the public assisted until emergency services arrived, police confirmed.
The firefighter was not on duty at the time.
A spokesman for the fire service said: “We can confirm that one of our firefighters was assaulted in Littlehampton on Monday night.
“Our thoughts are with him and his family at this time and we wish him a speedy recovery.
“We are absolutely appalled by this incident, and Sussex Police are currently investigating.”
Police enquiries are ongoing, and anyone who saw what happened is asked to report it online or call 101, quoting serial 1332 of 01/10.
Alternatively, visit the Crimestoppers www.crimestoppers-uk.org website or contact the independent charity anonymously on 0800 555 111.