The 500-kilogramme (1,100-pound) British bomb had been found on a construction site in Germany's financial capital on Thursday, the emergency services said.
A 700-metre (half-mile) evacuation radius was set up in the west of the city centre in an area that included a number of old people's homes, heating and internet infrastructure and facilities of the Deutsche Bahn national rail operator.
The work was expected to continue into the evening because of coronavirus restrictions, the authorities said.
Some 75 years after the war, Germany remains littered with unexploded ordnance, often uncovered during construction work.
Earlier this year, experts defused seven World War II bombs found on the future location of Tesla's first European factory, just outside Berlin.
Sizeable bombs have also been defused in Cologne and Dortmund this year.
In 2017, the discovery of a 1.4-tonne bomb in Frankfurt prompted the evacuation of 65,000 people — the largest such operation since the end of the war in Europe in 1945.