The prank appeared to be based on a tongue-in-cheek "apocalypse agenda" that was posted online and quickly went viral. The first item on the list, at 5 am, called for "waking up the world."
A police spokeswoman said official-looking flyers from a so-called "special protection office" had been making the rounds since Thursday.
The letters warned of "acoustic disturbances" throughout Jena the following day and advised residents to pay attention to "signal" alarms meant to alert them to danger, the Berliner Morgenpost reported.
On Friday morning, police started getting calls from residents complaining of loud beeping noises. The sonic trail helped authorities track down the source of the cacophony: alarm clocks that had been planted on the sides of homes in the area.
The pranksters hid the alarms in boxes labelled "signaller." Police, who described the sound as very loud and annoying, instructed residents to simply switch off the clocks.
Meanwhile, the pranksters behind the apocalypse ruse needn't worry about spending their last day on Earth in legal limbo - police said they are not planning to press charges.