A 500-kilogram World War II bomb was discovered during building work in Berlin on Thursday, forcing evacuations.
The bomb in the Friedrichshain district will have to be defused.
"There could be up to 12,000 people in the area of the exclusion zone, and it will take time," a police spokeswoman said.
An emergency shelter has been set up at the Mercedes-Benz Arena, which is normally used for concerts and sports events.
The 500-meter exclusion zone includes the busy Ostkreuz train station in the east of the German capital.
