Residents of the capital of the northern state of Lower Saxony had been told to leave their homes in the early evening on Monday as a precaution on the discovery of the 250-kilogramme (550-pound) device.

Tweets less than an hour apart from the city hall reported the bomb defusal team starting work and then issuing the all-clear at 01:07 am.

The unearthing of World War II era bombs is a common occurrence in Hanover, home to some 500,000 people and one of dozens of cities the Allies targeted during the conflict.

In June, one such device had to be made safe near Berlin's popular Alexanderplatz square.

Two years ago, some 65,000 people had to be temporarily evacuated -- the largest such operation since 1945 -- when a huge British bomb weighing 1.4 tons was discovered in Frankfurt.

Around one in ten of the millions of bombs dropped over Germany during the war did not go off, according to experts cited by news weekly Der Spiegel.

On Monday evening, a second unexploded bomb in the western city Cologne was also defused, after 4,800 people were evacuated.