A Shoreham shop assistant was attacked by one of three masked raiders.
They burst into a post office in Beach Road just after 4pm yesterday and grabbed hundreds of pounds.
The woman's injuries were not thought to be serious but she was taken to hospital.
The offenders, armed with blunt weapons, escaped in a getaway car.
Police sealed off the area and launched a search.
A number of people were in the post office at the time and police are keen to trace witnesses.
Sergeant Jason Wilson of Shoreham police said: "This is a serious incident. We are appealing for anyone who witnessed the raid or has any information to come forward."
It is the second time the post office has been raided this year.
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