More than 16 thousand people were evacuated from the Ostend area of Frankfurt on Sunday while experts began to defuse a 500 kilogramme World War Two bomb.
The site, near to the European Central Bank, was cleared of all people to allow the process to begin, taking visitors by surprise.
Tourist, Delilah Ahmed said: "The war was so long ago, and now something like this happened."
Her friend, Jasmin Ali, added: "And there's a bomb. And it's 2019 and they are still disposing, they're putting out bombs. So it's bit weird."
Bomb disposal professionals used specialist equipment to deactivate at least one of the two detonators.
Authorities moved some people out of a nursing home on Saturday in preparation for the process to begin.
Trains and cars were diverted and Frankfurt Zoo remained closed for the day.
75 years after the end of WWII, unexploded bombs are still often found in German.
The American bomb was found during construction work last month but officials chose Sunday to defuse it in order to minimise disruption to Germany's financial capital.