Police said anyone refusing to evacuate would be arrested.
In May, more than 50,000 people were asked to leave Hanover after a bomb was discovered during pre-construction work on a site in the Vahrenwald district.
Officials have ordered residents to evacuate homes within a 1.5-kilometer (nearly a mile) radius of the site in Germany's financial capital.
About 21,000 people have been ordered to leave their homes and workplaces in the western city of Koblenz as a precaution before specialists attempt to defuse the 500-kilogram (1,100-pound) USA bomb on Saturday afternoon. Others sat in cafes on the edge of the evacuation zone. If that fails, a water jet will be used to cut the fuses away from the bomb.
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The HC 4000 bomb is believed to have been dropped by Britain's Royal Airforce in World War II.
British and U.S. warplanes pummeled the country with 1.5 million tonnes of bombs that killed 600,000 people. An estimated 10 percent of the bombs failed to explode.
An average of about 2,000 tonnes of unexploded ordnance are found each year in Germany.
A police vehicle next to a blue tent covering a British World War II bomb in Frankfurt.
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The brigade's director, Reinhard Ries, told reporters: "The scale of this bomb is overwhelming".
Among those moved to safety are prison inmates and hospital patients.
Air traffic from Frankfurt airport could also be affected and small private planes, helicopters and drones were banned from the evacuation zone.
Museums were offering residents free entry today, and many hotels offered discounts.
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The national average for retail prices hit $2.468 per gallon Thursday afternoon, based on GasBuddy's live ticking average. USA gasoline futures spiked 14% to $2.163 per gallon in recent trading.