About 50,000 people in Hannover have been ordered to leave their homes while experts defuse five suspected World War Two bombs.
The operation is the second largest of its kind carried out in Germany, and will affect around a tenth of the city's population.
The buildings being evacuated include seven care homes, a clinic, and a Continental tyre plant.
Officials hope those affected will be able to return home by the evening.
The evacuation deadline was 09:00 (07:00 GMT) and residents were advised to take necessary items like medication with them, as well as turning off gas and electrical appliances.
Rail travel may also be hit by delays after early afternoon, The Local reported.
The city has set up a programme of museum tours, children's films, and sporting events to help people spend the day as pleasantly as possible.
Tens of thousands of soup portions are also being prepared, according to the German news agency DPA.
Bomb disposal experts had initially checked as many as 13 suspicious objects, but reports suggest only five were considered possible bombs - two on a building site at the city's Wedelstaße, and three others nearby.
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Allied planes bombed Hannover heavily during World War Two, killing thousands and destroying much of the city.
On 9 October 1943, an especially deadly night, 1,245 people were killed and 250,000 left homeless by 261,000 bombs.
The largest bomb-related evacuation since the war happened on Christmas Day last year, in Augsburg.
Some 54,000 people had to be moved after a 1.8 tonne bomb was unearthed during building work.
Other WW2 bombs recently discovered in Germany
- May 2015: 20,000 people in Cologne forced to leave their homes after a one-tonne bomb was discovered
- January 2012: A construction worker was killed when his digger hit an unexploded bomb in Euskirchen.
- December 2011: 45,000 people were evacuated from Koblenz after two bombs were found in the riverbed of the Rhine. It took three hours to make them both safe.
- June 2010: Three members of a bomb disposal squad were killed in Goettingen during an operation to defuse a bomb found on a building site.
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