More than 20,000 people are being evacuated from their homes in the west German city of Dortmund as authorities prepare to defuse a massive bomb left over from World War II.

The 4,000-pound (1,800-kilogram) bomb was discovered after experts analyzed old aerial photographs while searching for unexploded ordnance dropped by Allied aircraft over Germany's industrial Ruhr region.

City authorities said Sunday they expect the operation to defuse the bomb to be completed by about 3:30 p.m. (2:30 p.m. GMT).

While unexploded WWII-era bombs are regularly discovered in Germany, they are rarely as big as this one.

A similar bomb find two years ago prompted the evacuation of some 45,000 people in the city of Koblenz along the Rhine river.