Bomb disposal experts have defused an unexploded World War II bomb in the western German city of Koblenz, as a massive evacuation began in neighbouring Frankfurt to neutralise an even bigger explosive.

About 21,000 people had to leave the area in Koblenz as the 500kg American bomb was successfully defused on Saturday before the fire brigade allowed residents to return to their homes.

RELATED: 50,000 evacuate German city over unexploded WWII bombs

The operation came a day before three times as many people must leave their homes in Frankfurt to allow another bomb to be neutralised - the biggest evacuation of its kind in postwar Germany.

The HC 4000 bomb is assumed to have been dropped by Britain's Royal Air Force during the 1939-45 war.

The British bomb found in central Frankfurt weighs 1.8 tonnes and its disposal, planned for Sunday, has prompted a much larger evacuation protocol, affecting more than 60,000 residents.

Frankfurt fire and police chiefs said they would use force and incarceration if necessary to clear residents, warning an uncontrolled explosion of the bomb would be big enough to flatten a city block.

Delicate operation

The city said police will enforce a 1,500-metre radius evacuation zone starting early Sunday and residents will probably need to stay away until 8pm (18:00 GMT).

Seven decades after the end of World War II, unexploded bombs from intense Allied raids on Nazi Germany are still occasionally found in building sites, forests and fields, and sometimes even in private gardens.

More than 2,000 tonnes of live bombs and munitions are found each year in Germany.

In July, a kindergarten was evacuated after teachers discovered an unexploded World War II bomb on a shelf among some toys.