More than 54,000 people in the southern German city of Augsburg must leave their homes on Christmas morning while a giant 1.8-ton aerial bomb from World War II is defused.
The city's medieval cathedral and City Hall are in the area to be sealed off.
Police said no-one will be allowed into the surrounding streets from about 8am on Sunday and everyone must be out by 10am.
Police say it is impossible to say exactly how long it will take to make the bomb safe.
Schools are being opened for people who cannot stay with relatives or friends.
Police said people can bring their pets to shelters and that public transportation will be free on Christmas morning.
Finding bombs from the war is not unusual in Germany but this evacuation is even bigger than on the occasion when 45,000 people were evacuated temporarily to remove a bomb in Koblenz in 2011.
Large parts of Augsburg were destroyed on February 25 - 26 1944, when the city was attacked by hundreds of British and US bombers.