Police in riot gear descended on a small South Canterbury hall early yesterday after a party became so out of control that neighbours described the scene as "World War III".

"What we observed last night was perhaps the most frightening, alarming situation you could have in a small community," said resident Des Kearns.

"For us it's something we've only seen on television."

Otipua Hall association secretary-treasurer Jean Stewart said it began as a private birthday party for a 20-year-old man.

But word apparently spread through social media and other channels, and an estimated 200 people eventually showed up.

Police were called to the scene about midnight and were speaking to a few young people who had been threatened by other partygoers when individuals began throwing rocks and bottles at them, Sergeant Antony Callon of Timaru said.

"One of the young people was hit by a bottle," he said.

The officers at the scene stepped back and called for back up.

More than a dozen police equipped with riot gear eventually waded into the crowd to gain control.

Police remained at the scene until about 3am, he said.

"It was like World War III out here," Kearns said. "Imagine, in Otipua, police going against a crowd with shields and helmets and truncheons."

Eight people, males ranging in age from 15 to about 21, were arrested in the incident, Sergeant Callon said.

Most were charged with disorderly behaviour likely to cause violence after they were identified as throwing bottles and rocks at police and police vehicles.

No serious injuries were reported.

When the crowd dispersed, the hall and carpark was in total disarray and covered in rubbish, Stewart said.

"There was blood on a curtain and windowsill and a curtain rod was pulled down where it looked like someone had tried to go out a window," she said.

Beaconsfield Rd was so littered with broken glass that cars could not drive through.

A group of cyclists on a training run early yesterday suffered multiple tyre punctures.

Otipua residents pulled together to begin cleaning up the mess.

Eventually a Timaru city street sweeper drove out to finish the job.

Party organisers had one adult there to keep an eye on the door, who did their best to control the situation, Sergeant Callon said, but the trouble apparently began when a group of uninvited people turned up.

"When you're inviting in excess of 50 or 100 people, there's always going to be some antisocial element turning up to these events who seem to thrive on violent behaviour," he said.

"It's a very small group of delinquents who drink too much alcohol and take satisfaction in trying to use violence against not only members of the public but also law enforcement."

Anyone with information about violence at the event is asked to contact Timaru police on (03) 687 9808.

- © Fairfax NZ News