A party invitation that went viral on Facebook ended in rioting and injury when thousands of revellers descended on a small town in the Netherlands.
Riot police broke up crowds of youths who turned violent in Haren, north of Amsterdam, Reuters reports.
Media reports said six people were hurt, including three seriously, in the disturbances.
Shops were vandalised and looted before the police broke up the crowds. There were 20 arrests.
Dutch media said 30,000 people mistakenly received the invitation from a girl announcing her birthday party on Facebook.
The girl whose 16th birthday was being celebrated had not set her Facebook event to 'private'.
Dutch television said up to 4,000 mainly young people had turned up in Haren, which has a population of 18,000.
Cars and doorways were damaged by objects thrown at police blocking the street where the girl lives. The teenager had fled her home earlier in the day.
Several websites had sprung up dedicated to the party and referring to 2012 US teen film, Project X, about a suburban birthday party getting out of control after an invitation goes viral, the ABC says.