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 bomb from the Second World War.

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 moved Saturday, Frankfurt city Coun. Markus Frank told Reuters television.

Over 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 WW II bomb on a shelf among some toys.

Germany WW II Bomb

A blue tent covering the bomb is seen on Friday. It was discovered on a building site in Frankfurt’s leafy Westend, where many wealthy bankers live. (Boris Roessler/dpa via AP)

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 pummelled by 1.5 million tonnes of bombs from British and American warplanes that killed 600,000 people. German officials estimate 15 per cent of the bombs failed to explode, some burrowing six metres deep.

Three police explosives experts in Goettingen were killed in 2010 while preparing to defuse a 450-kilogram bomb.

Germany WWII Bomb

Numerous ambulances wait outside the Buergerhospital on Saturday. (Frank Rumpenhorst/dpa via AP)

The compulsory evacuation radius of 1.5 kilometres around the bomb includes police headquarters, two hospitals, transport systems and Germany’s central bank storing $70 billion US in gold reserves.

Frankfurt’s residents have to clear the area by 8 a.m. local time 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 defusing 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 centre. Most museums are offering residents free entry on Sunday, and a few of them will open their doors earlier in the morning than usual.