Evacuation of some 70,000 people began on Sunday in Germany's Frankfurt city in order defuse a massive unexploded World War II bomb.
The bomb, which was dropped by the British during Allied bombing campaigns, is an aerial mine HC 4000, weighing 1.4 tonnes, reports Efe news.
The evacuation is set to last two hours, the police said.
A security perimetre of 1.5 km was set up around the area where the bomb was found on August 29 near Goethe University in Westend, which is home to the Bundesbank (Central Bank of Germany), police headquarters, the headquarters of Hesse radio and television broadcasting, Palmengarten botanical gardens, two hospitals and 20 retirement homes.
A thousand firefighters and an undetermined number of police officers were assisting in the operation, the largest in Germany's post-war history.
Unexploded bombs are regularly found in German cities. Allied forces carried out a heavy bombing campaign, and between 10 per cent and 30 per cent of the bombs that were dropped did not explode.
Some 2,000 tonnes of undetonated explosives continue to be located every year in Germany, more than 70 years after the end of World War II.
On Saturday, 21,000 people were also evacuated in Koblenz, and earlier this week the deactivation of another device in Berlin forced the closure of Tegel airport for several hours.