A cordon of 150m (492ft) has been established following the discovery of suspected unexploded ordnance in Falmouth Docks.
Devon and Cornwall Police said emergency services were called at 09:15 BST after the object was found.
The cordon covers both land and sea, and two homes have been evacuated while an investigation is carried out.
Specialist explosive ordnance disposal officers have been contacted. Members of the public and boat users are asked to avoid the area, police added.
A police spokesperson said: "There is currently no information to suggest any wider risk beyond the established cordon. However, the public is urged not to approach the area or enter any restricted zones."
Crews from Cornwall Fire and Rescue Service, the coastguard, the port authority and the harbourmaster are also at the scene.
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Suspected Unexploded Ordnance at Falmouth Docks Prompts Cordon