Emergency service staff have evacuated patients from two hospitals in Germany's financial capital.
Some 60,000 people have to leave their homes early on Sunday in Germany's biggest evacuation since the war while officials disarm the 1.4 tonne British bomb.
It was discovered on a building site in Frankfurt's leafy Westend, where many wealthy bankers live
More than 2,000 tonnes of live bombs and munitions are found each year in Germany, even under buildings. In July, a kindergarten was evacuated after teachers discovered an unexploded World War Two bomb on a shelf among some toys.
Frankfurt fire and police chiefs said they would use force and incarceration if necessary to clear the area of residents, warning that an uncontrolled explosion of the bomb would be big enough to flatten a city block.
The HC 4000 bomb is assumed to have been dropped by Britain's Royal Air Force during the 1939-45 war.
Frankfurt's residents have to clear the area and police will ring every doorbell and use helicopters with heat-sensing cameras to make sure nobody is left behind before they start diffusing the bomb.
Roads and transport systems, including the parts of the underground, will be closed during the work and for at least two hours after the bomb is defused, to allow patients to be transported back to hospitals.
Air traffic from Frankfurt airport could also be affected if there is an easterly wind on Sunday. Also, small private planes, helicopters and drones will be banned from the evacuation zone.
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