Builders working at the site in the southern suburb of Frölunda found the suspicious object at 10am and immediately informed the police, after which officers evacuated all those who were still at home out of the 65 people registered as living in the building.
“We suspect it is a home-made bomb, and that it might be quite powerful,” Thomas Fuxborg, spokesman for the police in western Sweden told the Göteborgs Posten newspaper.
Fuxborg said it could have been at the site “a matter of hours, or a few days, but not more than that”.
Police said they had detonated the object just after 1pm.
Fuxborg's colleague Tommy Nieman told The Local at 2.30pm that the police were still waiting for bomb technicians to come to their conclusions.
“We can't say any more at present other than that we are waiting for the results of the investigation and we are waiting for the technicians,” he said.
Several of those evacuated have taken shelter at the nearby Tynnereds church.