FRANKFURT (Reuters) - Frankfurt emergency service staff started to evacuate patients from two hospitals in Germany’s financial capital on Saturday ahead of the planned defusing of a massive World War Two bomb.

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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 100 hospital patients, including premature infants and those in intensive care, were evacuated on Saturday, Frankfurt city councillor Markus Frank told Reuters television.

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.

The country was pummeled by 1.5 million tonnes of bombs from British and American warplanes that killed 600,000 people. German officials estimate 15 percent of the bombs failed to explode, some burrowing six meters (yards) deep.

Three police explosives experts in Goettingen were killed in 2010 while preparing to defuse a 1,000 lb (450 kg) bomb.

The compulsory evacuation radius of 1.5 km (roughly a mile) around the bomb includes police headquarters, two hospitals, transport systems and Germany’s central bank storing $70 billion in gold reserves.

Frankfurt’s residents have to clear the area by 8 a.m. (0600 GMT) on Sunday 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.

Frankfurters can spend the day at shelters set up at the trade fair and the Jahrhunderthalle convention center. Most museums are offering residents free entry on Sunday, and a few of them will open their doors earlier in the morning than usual.