"It is safe to say that had the bomb detonated it would not only have caused material damage but people could also have been hurt," Kjell Lindgren of the Stockholm police told news agency TT.
The device was found in a box with the word "bomb" written on it. It had been placed outside the stadium on Thursday afternoon, just hours before a planned evening game designed to test the match readiness of the venue. The match was cancelled.
The motive is unclear and police have refused to speculate whether it was connected to the friendly match that was due to take place between two veteran teams from the Stockholm club Djurgården, which will share the venue with the Hammarby football club, also from Stockholm.
"Stockholmers have paid for the arena, anyone else is a tenant. We're assuming that the two football clubs will do their utmost to make it work," said the Liberal Party's Madeleine Sjöstedt, cultural and real estate commissioner for Stockholm, at a press conference Friday afternoon.
Police said they will continue to search the arena grounds on Friday using sniffer dogs. There is currently no suspect in the case.