The brawl took place at the Youngstorget square in the Norwegian capital and a machete and other weapons may have been used in the fight, VG reports.
Around 30 people thought be in their twenties and thirties are reported to have attacked each other with various objects.
“We received the first report at around five minutes before 1am of a so-called mass brawl. There was a large group of men described as men with African backgrounds reported to be fighting. A machete was mentioned but we have not found that so far. Most appear to have used iron rods, wooden planks and bottles,” police response leader Tor Jøkling told VG.
Jøkling added that people involved in the fight dispersed in all directions when police arrived at the scene but three individuals have so far been apprehended.
Shortly after, police were informed of an injured woman at a bar in nearby Torggata.
The woman was reported to be bleeding and close to fainting.
“We have received confirmation that there was a guest at a nightclub who was inadvertently hit in the head by an object that was thrown. An ambulance is seeing to her and she is in good care. We are treating this as related to the fight and are working on finding those responsible for this,” Jøkling told VG.
Police are currently looking for witnesses and persons who may have been involved in the incident.
“We must stress that there were so many people involved that it is a challenge for our patrols to get an immediate overview,” Jøkling continued.
Three people had however been detained in connection with the incident by 2:21 am, police said, and Jøkling later confirmed that one of these was a primary suspect in relation to the incident.