An explosive device was found outside Stockholm's new Tele2 Arena, a ground which will be shared by local rivals Hammarby and Djurgarden from next month.
The two clubs have been summoned to a crisis meeting after the device was found on Thursday. A Djurgarden masters team was preparing for a charity match at the time, which Hammarby fans interpreted as move to claim the ground by hosting the first match there. The match was subsequently cancelled.
"According to our technicians, it was operational and not a dummy," police spokesman Sven-Olof Olsson told reporters.
Hammarby will play the first official match at the 30,000-capacity ground when they host Orgryte on July 20 in the Swedish second division.
Djurgarden, who play in the top division, the Allsvenskan, rejected claims that they organised the charity match to hijack the inauguration of the ground.
"It was a test event," the club's communications manager Gunnar Gidefeldt told Sportbladet. "We did not inaugurate the stadium. A premiere is supposed to be an important football game."