Allsvenskan clubs Hammarby and Djurgarden have been summoned to a crisis meeting after a bomb was found outside the new stadium the clubs are due to share.
An explosive device was discovered on Thursday prior to a charity match at the new Tele2 Arena in Stockholm, in which a Djurgarden masters side was due to compete.
"According to our technicians, it was operational and not a dummy," police spokesman Sven-Olof Olsson told reporters in Sweden.
Disputes had surfaced between sets of supporters ahead of the match, with some Hammarby fans claiming its rival was attempting to hijack the opening of the new ground before the first official clash between Bajen and Orgryte on Saturday.
Djurgarden strongly denied the suggestion, insisting the charity match was simply a useful test for the new venue.
"It was a test event," Djurgarden's communications manager Gunnar Gidelfeldt told Sportbladet. "We did not inaugurate the stadium. A premiere is supposed to be an important football game."
The match was cancelled after the device was found, and both clubs have since been called to an emergency meeting with police.