A dozen people were arrested and a man smashed an ambulance window, covering a paramedic in glass, at a Dunedin street party tonight.

Some 4,000 tickets were sold for the Hyde St keg party, an annual student blow-out known for its excessive alcohol consumption.

In a statement released about 8.30pm, police said a dozen people had already been arrested for offences including assault on security staff, fighting, disorder and offensive behaviour.

A particularly distressing incident involved an intoxicated male who damaged three vehicles including shoulder-barging the windscreen of a St Johns ambulance. The female paramedic inside was covered in shattered glass fragments.

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"This is yet another example of the harm alcohol causes and the poor decision-making of those under the influence," Inspector Mel Aitken said.

"Despite the best efforts of organising staff to create an enjoyable occasion for students, the level of alcohol related harm and the risk to public safety still remains too high."

The party was "far from being what police deem a safe event".

Last year's event saw 10 people arrested.