An ambulance crew was forced to leave the scene of a fight in Hamilton after teenagers began pelting their vehicle with bottles.
Police say they were called to Hammond Street in Hamilton West, when neighbours became scared of about 200 people smashing bottles along the street after a party there.
People at an events centre also called police and were told to stay inside.
When police arrived, they were also attacked with bottles.
Extra backup was called and as many as 30 officers were involved at the height of the incident at 1.15am on Sunday.
A St John Ambulance crew left the scene because of the danger but no emergency personnel were injured.
St John's central region operations manager Grant Pennycook said the ambulance had parked a safe distance from the party.
He said party goers approached it after being dispersed by the police, which is when the bottles were thrown.
Mr Pennycook said the ambulance left immediately, and no staff were hurt in the incident.
He said they work very closely with the other emergency services and are happy with the way the police handled the situation.
Four people were arrested; one for assault with a weapon.
They will appear in court next week.
Police said on Sunday they had been warned about the possibility of trouble at the party after a large number of RSVP's on Facebook.
Police visited the house at 10pm and asked partygoers to be quiet and get off the street.
Inspector Karen Henrikson said the party was out of control by 11.30pm.