Nine teenagers were arrested in Nelson after the city's annual Masked Parade on Friday.
Police say they broke up numerous fights involving between 30 and 60 people and are investigating an assault by two girls on another and damage done to a car.
Nine youths aged between 14 and 18 were arrested for disorder offences between 6.30pm and midnight.
Up to 300 young people had congregated around the Cathedral Steps after the parade, police say.
Sergeant Stu Koefoed says the problem of disorder after the event has grown over the past three years.
Thousands of people had earlier marched and lined the streets to watch the parade, which marked the opening of Nelson's Arts Festival.
The festival's artistic director Annabel Norman says disorder this year seems to have escalated and the behaviour of some teenagers is jeopardising their participation in the event.
Ms Norman says many teens have been coming to the Masked Parade since they were young, and it's become a tradition for them to come to the city in groups now they're older, and drink alcohol.
She says a team of youth workers liaises with the teens during the festival, as well as police and ambulance staff.
Nelson mayor Kerry Marshall says thousands of people turned out to watch the parade and it's a shame some teenagers chose to take advantage of the environment by drinking and causing trouble.