Police were called to the Co-op store, in The Square, at around 3pm, following reports of an armed robbery at the site.
More than a dozen police officers, alongside a number of specialist armed response teams and a dog unit have since cordoned off sections of the village, while scenes of crime officers conduct their search.
Jon Wheatland, a chef at The Lamb, opposite the Co-op, saw the armed man run past the public house, minutes after the alleged crime.
He said: “It was about 3.05pm and this man ran past the pub wearing a gas mask with – what I thought – was a paintball gun.
“I didn’t think anything of it at first but then I saw him holding a brown bag and it just clicked, I guess.”
The suspected robber was then seen running into St Nicholas’ Gardens, behind the pub.
It is unclear yet how much cash – if any – was taken from the Co-op.
Inspector Jason Wilson is now urging any witnesses to the incident to come forward and contact police.
He said: “We’re investigating reports that a white man, with some sort of mask – possibly armed with a hand gun – robbed the Co-op store, in The Square.
“At the moment we’re following various lines of inquiries in order to track down the male involved and would appeal for anyone who may have witnessed the crime, to come forward and call us.”
Anyone with information is asked to contact Sussex Police on 101 quoting serial 1084 of 21/8 or the independent charity Crimestoppers anonymously on 0800 555 111.