Several town centre shops were evacuated after a hand grenade was found in the basement of an estate agents.
Police cordoned off part of East Grinstead High Street after the unexploded device was discovered lodged beneath a pipe at about 1pm.
Officers said staff made the discovery while they were clearing out the basement to move to a new premises.
Its safety pin was still in place and the area has been taped off while emergency services wait for the Explosive Ordnance Disposal experts to arrive.
On Twitter, John Pye shared a picture of the scene and said: “Police evacuated and cordoned off your favourite local bookshop today, as an unexploded grenade was discovered in the former Coles estate agency’s cellar.”