The German city of Cologne has finished evacuating 20,500 people, its largest evacuation order since World War II, after officials defused three massive, unexploded bombs.

The American bombs – two 20-tonne weapons and another that weighs 10 tonnes – were found in a shipyard on Monday, the city said, causing a huge “danger zone” to be sealed off early on Wednesday.

A hospital, two retirement centres and the city’s second-largest train station were among the facilities emptied out. Schools, churches, museums and two of the city’s cultural landmarks – the Musical Dome theatre and the Philharmonic Hall – also fell within the evacuation zone.

The discovery of unexploded weapons is a frequent phenomenon in Cologne, which was decimated by Allied bombing during World War II, but no operation of this size has been carried out since the end of the war, the city said.

“At 7.19pm, the three unexploded bombs that had paralysed large parts of the city were defused. Experts from the ordnance disposal service Rheinland of the Düsseldorf district government rendered them harmless,” city authorities said in a statement.

Streets and buildings are being gradually reopened, and residents can return to their homes and apartments, the authorities added.

Allied nations conducted 262 air raids of Cologne during World War II, killing approximately 20,000 residents and leaving the city in ruins.

Nearly all of the buildings in the Old Town were destroyed, as were 91 of the city’s 150 churches.

A massive reconstruction effort took place after the war, with the Old Town rebuilt and major landmarks restored.

But small evacuations still take place on a regular basis when unexploded ordnances are found. About 10,000 residents had to leave their homes last October when another American bomb was found, and last December, 3000 people were asked to evacuate.