Tokyo Shoumetsu Sensou

They were instructed to kill each other in the city of Tokyo until 10,000,000 people were eliminated from the city. This genocidal game known as "Save the Earth" happens every 4 years. An over-populated city is randomly chosen to become the forced killing grounds of its denizens, and each person has to kill the other. They have no choice, and no escape. Akira Suwa must now fight to protect his loved ones and himself in this genocidal game.

(Source: MangaHelpers)