Two people were hospitalised after two brawls in Turku early Saturday.
Southwest Finland Police said one of the fights broke out around 1.30 am at a bar on Linnankatu.
According to the restaurant staff, some 20-30 people took part in the fight. Some were in possession of knives.
Police said the venue's security guards were unable to contain the situation, and that its security arrangements were completely insufficient.
Police emptied the restaurant and ordered it to close. An investigation was underway on Saturday, including a review of the venue's alcohol license.
Based on the preliminary investigation, the confrontation involved a settling of scores between two or more groups. Some participants had bladed weapons but there were no reports of serious injuries.
Two people were hospitalised after another dispute involving bladed weapons, which took place later in the night on Aurakatu. It is near the River Aura, which bisects the port city and features many boat restaurants.
Southwest Finland police said they responded to more than 300 incidents during the hot night, several of them involving violence.
A number of individuals were detained on suspicion of crimes and for aggressive or disruptive behaviour.
Stabbing in Pori, unruly crowds in Hanko
In Pori, further up the west coast, there was another violent confrontation involving a bladed weapon. A man was stabbed on the city's Market Square shortly before 2 am, sustaining potentially life-threatening injuries.
Further south in mainland Finland's southernmost town, the Hanko Regatta began on Friday for the first time since 2019. The event has often attracted large crowds of young people for rowdy partying.
In a tweet, Western Uusimaa Police said that they were monitoring crowds who were "on the move in large numbers" and had dealt with numerous cases including aggravated assault, brawls and disruptive behaviour.
An Yle reporter on the scene said there was widespread unruliness overnight, with large crowds of unmasked people and streets littered with broken glass. The Hanko Regatta wraps up on Sunday.