As many as 400 people turned out for the 16-year-old's private celebration near the southwestern city of Mannheim. Police took several hours to restore order after the drunken crowd turned violent.
A 16-year-old's birthday party turned from a family affair into a ugly rampage on Friday night when up to 400 uninvited guests descended on the celebration.
Police in the town of Grünstadt, around 30 kilometers (19 miles) from Mannheim, said the party got entirely out of control as many of the gatecrashers were drunk and behaving aggressively.
The birthday girl and her father were overwhelmed by the arrival of so many uninvited guests to the community center where the event was being held. The amount of noise and disturbance prompted several neighbors to call the police.
When officers arrived, they described a heated mood, with many of the partygoers acting verbally aggressive.
When they tried to call off the party, the situation quickly descended into a mini-riot. Some of the crowd threw stones at officers, who then used truncheons to fight them back. One policeman was injured when a rock hit his head.
Police said it took several hours to bring the situation under control. Reinforcements from nearby towns were called in to help.
Nine people were arrested and could face charges of public disturbance, insulting officers, resistance, and serious bodily injury.
The birthday family says it only invited relatives and close friends to the event.