A party invitation which went viral on Facebook ended in rioting and injury after thousands of revellers descended on a small town in the Netherlands.
Haren had been braced for trouble all week after what should have been an invite to a small-scale celebration was passed on to 30,000 people.
The girl whose 16th birthday was being celebrated had not set her Facebook event to “private”.
Riot police broke up crowds of revellers who flocked to the town.
The girl who issued the invitation fled her home in Haren, a town of just under 19,000 near the city of Groningen, on Friday.
The party had been cancelled and police had issued an appeal to would-be revellers not to come to Haren but at least 3,000 turned up anyway.
“She posted the invitation on Facebook and sent it to friends, who then sent it to other friends and soon it spread like wildfire across the internet,” Groningen police spokeswoman Melanie Zwama told AFP news agency.
Hundreds of riot police were deployed to control the crowds, keeping them away from the street where the girl lives.
When trouble began, officers found themselves being pelted with bottles and stones, as well as flower pots even bicycles, the Dutch news agency ANP reports.
At least six people were hurt and 20 arrests were made as rioters vandalised and looted shops, setting a car on fire and damaging street signs and lamp-posts, according to Reuters news agency.
Some revellers accused the police of over-reacting.
Elsewhere, Dutch media were accused of giving the build-up to the party too much publicity.
Revellers could be seen wearing T-shirts marked “Project X Haren” after Project X – a film released earlier this year about a party that grows out of control.
Such T-shirts had been selling on the internet for 23 euros (£18; $30) apiece. Some featured a crude logo of a man on all fours drinking from a bottle, AFP notes.
A new Facebook page has since been created called “Project Clean-X Haren” to clear up after Friday night’s disturbances. It had more than 17,000 “likes” as of Saturday morning.