The alarm about an ongoing assault at a residence in Visby came in to the police just after half past three on Friday night.
There is an injured man at the scene, who is taken over by paramedics and taken to hospital for treatment. It is not known how seriously injured the man is.
The men are familiar
Another man at the scene is suspected of having stabbed the injured person, and is arrested on suspicion of attempted murder.
According to the police, the two men are familiar with each other.
The police have launched an investigation, and are now knocking on doors in the area to try to get more information. A technical examination of the crime scene must also be carried out.