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A man in his 60s was badly injured when "according to witnesses he picked up an object off the ground which promptly exploded," police spokesman Sven-Erik Olsson told AFP.
A woman aged 45 was also hurt, police said.
The blast occurred mid-morning at the Varby guard station in Huddinge, a southern suburb of the Swedish capital.
Police cordoned off the station and the square where the blast happened as the bomb squad moved in to investigate.
The Expressen and Aftonbladet newspapers said the device was a hand grenade.
"It is too early to say. Technicians are still working on it. Nothing indicates that the (injured) couple were targeted," said Olsson.