A 40m (130ft) newly-built section of the bridge, as well as scaffolding around it, fell 20m onto workers, said police spokesman Bjorn Schmitt.
It was not immediately clear what caused the accident, which happened on Wednesday near the town of Werneck.
There are thought to have been around 20 workers on the site at the time.
Initial police reports at the scene of the collapse in northern Bavaria suggested at least two people had been killed.
Mr Schmitt said he was concerned that workers were buried under the wreckage of the scaffolding.
Authorities are trying to establish if any cars had been travelling along the country road which passed underneath it at the time.
Traffic on the A7 road, which still uses the old undamaged bridge that is being replaced, was not affected by the incident, Deutsche Welle reported, although temporary speed restrictions are being enforced around the accident zone.
Work on the new 236-metre (774ft) bridge started last year and was supposed to be completed by the end of 2017.
The bridge is more than 50 years old.
A scaffolding collapse on a bridge being built in Germany has killed at least one worker and seriously injured several others, police say.