At least 70,000 people in the Greek city of Thessaloniki are being evacuated so that a 500lb World War Two bomb can be defused, officials say.
It is thought to be one of the largest wartime bombs to be found in urban Greece in addition to being one of the largest mass evacuations.
The bomb was discovered during road works last week and is due to be disposed of on Sunday.
Officials say it is too degraded to tell if it is German or an Allied bomb.
Residents within a radius of about 2km (1.2 miles) of the bomb will be compelled to evacuate the area between now and Sunday morning, security officials have said.
The operation has been described by one blog as the biggest evacuation of Greek civilians in peacetime. However, it is not possible to verify such a claim.
The military says an operation of this size and complexity is the first of its kind in a densely populated area of Greece and the disposal operation should take about eight hours – but may take as long as two days.
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