German authorities have evacuated a downtown area of the eastern city of Dresden after an unexploded World War II-era bomb was found during construction work.

Authorities say around 8,700 people were evacuated from homes and businesses in an area near the main train station Wednesday as experts worked on defusing the 250-kilogram (550-pound) British bomb found the day before.

Officials say the bomb has proved a tricky one to defuse and it wasn't clear how long the operation would take.

Much of Dresden was turned to rubble by Allied bombing during World War II, immortalized in Kurt Vonnegut's famous novel "Slaughterhouse-Five."

More than 70 years after the end of the war, bombs are still regularly discovered in German cities during construction work.