It was at 01.25 that the police alerted a restaurant in central Trosa.
There, five men, all in their 20s to 25s, are said to have abused two men in their 40s and 45s.
- One man must have lost consciousness and wet his face thoroughly, which makes us look at it as aggravated assault.
The other man's injuries are not as serious, but he must also have been injured, says police spokesman Åsa Willsund.
The seriously injured man was taken to hospital by ambulance.
Five perpetrators based in Stockholm
When the police arrived at the scene, the perpetrators had managed to leave by car.
Witnesses had seen the car's registration number and police were able to stop it just minutes after the alarm.
The five people in the car, all residents of the Stockholm area, were arrested on suspicion of aggravated assault.
According to the police, the perception is that the perpetrators and the victims do not know each other but meet for the first time in the pub environment.
- Exactly how everything went, I do not know yet.
We are currently questioning the perpetrators.
One has already been questioned and arrested for aggravated assault.