Officers said they were called just before 9am to a flat in Fairbank Road, in Southwater. On arrival they found a device which they were concerned may be an explosive.
A 100-metre cordon has been set up around the car park in Lintot Square and nearby homes and businesses have been evacuated.
Police said a bomb squad from Aldershot was currently in attendance.
Horsham District Council is urging people to stay away from the area.
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