Nobody was injured in the blast, which happened at 1am at the street Gyllenstiernsgatan in the Östermalm area of the Swedish capital, but it could be heard several kilometres away.
It damaged the building and cars parked along the street, with broken glass and debris covering the area.
In response, police launched a so-called “special incident” (or särskild händelse in Swedish), a move that usually means a temporary task force is set up to focus solely on the specified problem, and which can be launched to deal with a range of unexpected or sudden issues for which more resources are needed.
Late on Monday it remained unclear exactly what caused the explosion, which is being investigated as a criminal act, or specifically “devastation endangering the public” (allmänfarlig ödeläggelse).
No arrests had been made, but police said they had received a lot of tips and information.
There were no known threats that had been made against any resident in the building, which is located in one of the most expensive areas in central Stockholm, but police said they were still investigating.
“We have extra resources on the site to be available to residents and others who have questions or are feeling anxious about the incident,” Erik Widstrand, head of the Stockholm City policing area, said on Monday.
“It is one of the most powerful explosions we have had in the region and we are keen to collect as many observations and tips as possible.”