Disu Ma Topia

From DrCoke at MangaHelpers:

The story begins in the near future, where a social mobile network called the "Kizuna-Topia" (Bonds-Topia) controls all communication and surveillance in the wall-bound city.

In this city, two sisters, Enna and Yayako, come face to face with the ongoing man-hunt for young, healthy children. The kidnappers take these kids to the white tower to extract their brains and sell their bodies. This man-hunt, led by the local police known as the "Jikei," is counteracted by the mysterious being only known as "The Witch."

Enna, Yayako, and other residents of this city are trapped between the ongoing conflict between the Jikei and The Witch.