Swedish Radio reports that the new bridge, due to be complete in October, was supposed to carry heavy-duty traffic, replacing an old bridge built in 1905.
It collapsed onto an active railway track where trains pass towards Falun, Gävle, and Hallsberg.
According to details given by SOS Alarm to Swedish Television, the bridge collapsed when a foundation beam gave in just after 14:00 on Thursday.
“The rescue services are sending all they have,” Stefan Wickberg, a police spokesperson, told Swedish news agency TT.
The most recent police report has twelve people injured, four of them seriously.
Martin Mullback, head of the police task force, has said that the collapse will be investigated as a workplace accident.
"We are now beginning the investigation. This means that the area will be shut down and treated as a crime scene to be analysed by technical experts," he told TT. "None of the injured are left at the scene. As of now, it is only people who worked on site that have been injured."
Passengers are advised to contact their rail operators for more information.