LAUNCESTON police resources were put under pressure after a party advertised on Facebook spiralled out of control.
One policeman also sustained a minor knee injury after an altercation with an unruly offender.
Police were called to an out-of-control party in Canning Street, Launceston, about 10.40pm on Friday.
The gathering was in a small unit near Brickfields Reserve.
About 200 people were spilling out of the residence and into the street when the first officers arrived on the scene.
They immediately called for back-up.
Sergeant Ivan Radosavljevic, of the Northern Road and Public Order Services unit, attended the incident.
``It was a strain on police resources, because we had to get officers from all over the district to attend and assist,'' he said.
``In excess of 20 police had to attend. We were well outnumbered. It was one of the worst I have seen in a while.''
Sergeant Radosavljevic said the youths were very anti-social, aggressive towards police and left scores of broken alcohol bottles and rubbish in the street.
Police used capsicum spray to control the crowd.
Police arrested nine people, including youths and adults, and have charged three more youths with various offences.
Allegations include assaults against police, disorderly behaviour and alcohol possession in a public place.
Sergeant Radosavljevic said it was becoming more common for police to be called to out-of-control parties.
He advised people not to advertise parties on Facebook and warned that organisers of rowdy parties and adults who gave alcohol to youths could be charged with offences. Police expect to charge the Canning Street party organisers with various offences.
Sergeant Radosavljevic encouraged people to register their parties with police through the Party Safe Program at www.police.tas.gov.au/services-online/party-safe/.
The out-of-control Launceston party follows one advertised on Facebook in the South last week, when police had to control up to 100 teenagers who were drinking alcohol in a Taroona house.