A total of 18 people were attacked by the hornets at the community festival, held to celebrate the end of summer, in the town of Weingarten in Karlsruhe, BILD reports.
Emergency services were called to the scene on Sunday. Thirteen people from the group had to be taken to nearby hospitals for medical treatment, the police said.
The hornets were gathered in their nest in a tree trunk but it's not known what triggered the stinging frenzy. Hornets can be dangerous, particularly to people who suffer from allergies.
No children were hurt in the incident and none of the injuries are thought to be life-threatening, according to reports.
Emergency teams helped to ease the panicked crowd. A hornet specialist was called in to ensure the insect nest was properly removed, and the festival area was closed off for the rest of the day.
Hornets, which belong to the family of wasps but are slightly larger, are a protected species in Germany and their nests may only be moved with official approval.
Both hornets and wasps are very common insects during summertime in Germany. People are advised not to make sudden movements around them or disturb nests to reduce the risk of receiving a nasty sting.