Police were called to close down a teen house party which got ‘out of hand’ in Bognor on Saturday night when a large number of uninvited guests arrived.
At one stage more than 400 people were estimated to be present at the party at a property in Chalcraft Lane, Bognor Regis, police said.
Details posted on social media enabled many young people who were unknown to the host to turn up uninvited, according to police.
Police said many of the people present were intoxicated.
Officers were drafted in from further afield to support local police in closing down the party.
The road was closed for two hours while the youths were dispersed, police said.
No serious injuries were reported, but one girl was treated in hospital after having apparently taken drugs, confirmed police.
Police said four arrests were made during the dispersal.
Inspector Nick Bowman said: “The party presented a huge challenge for police and created significant noise and disruption, not only for immediate neighbours but also those beyond.
“Youths were seen running over parked cars, some threw things at police and others chanted obscenities while behaving in a generally loud manner.”
Officers returned to the area on Sunday to provide reassurance to residents disturbed by the incident, he added.