A man and woman who were inside the first floor apartment at the time of the blast escaped unharmed. With the aid of emergency services they were able to climb down from their balcony to safety.
According to police inspector Daniel Hedman, an explosive device had been attached to the door of the apartment.
Police said the blast at the apartment block in Sundbyberg, a municipality in north Stockholm County, had left a football-sized hole. There have so far been no arrests.
The explosion caused extensive damage to the building, which was guarded by police on Tuesday morning.
A neighbour raised the alarm shortly before 3am. Six police units were called to the scene to question possible witnesses.