Buses are collecting people from the eastern town of Bialystok and neighboring Suprasl County, and taking them to schools where they will wait until the bomb is safely removed.
Anna Kowalska, spokeswoman for Bialystok town hall, told The Associated Press on Sunday that the evacuation was underway.
She said that about 10,000 people are registered as residents in the affected area.
The 500-kilogram (1,102-pound) German ST-500 bomb was found Thursday. Bomb disposal experts will load it on a special truck and destroy it at a test range.