What’sthat going bump in the…day? At the moment, downtown San Francisco is witnessing the arrival of hoards of ghoulish zombies.

Don’t panic! They’re hosting a zombie “blood drive” and handing out some “blood pops” (yummy hibiscus mint-flavored pops) to curious citizens and passerby on the street.

Their arrival is all in celebration of the new mobile social game from Zynga, Zombie Swipeout, where the object of the game is to slash, slice, bludgeon, explode, and freeze as many zombies as possible without harming Joey, the lone human survivor. 

The zombies took over our SoMA office already where screaming, banging on windows, and shrieking while running away ensued. One of our colleagues locked himself inside an office to avoid the freaky mob. The zombies were en route to meet other zombies in South Park for the lunch hour. Even more zombies will be coagulating in Union Square at the same time. Expect the whole horde of undead today at the Ferry Building around 2 p.m.

Zombie Swipeout is set to go live globally in the coming weeks on iOS (iPhone and iPod Touch). Read more about it here.