French authorities are evacuating tourists and others from the Mont-Saint-Michel abbey and monument in western France after a visitor apparently threatened to attack security services.
The national gendarme service said the man made the threat on Sunday on one of the shuttles serving the site, on a peninsula in the English Channel.
An official with the gendarme service said the threat was vague, but prompted authorities to order an evacuation.
Police are searching for the man who made the threat.
A shuttle service official said large crowds are being evacuated or turned away and a police helicopter is flying overhead, but the atmosphere is calm.
The site is one of the most visited in France, with several thousand tourists a day.