The nearly 150m-long bridge connects the west bank of the Gudbrandsdalslaagen River and the village of Tretten. The bridge opened in 2012.
"It is completely catastrophic, completely unreal," local mayor Jon Halvor Midtmageli told the Dabgladet newspaper. "It is also a fairly new bridge.
"It is completely destroyed, everything has fallen down," he added.
The Norwegian Automobile Federation said the bridge was checked in 2021, raising concern about the safety of such bridges.
"We who travel on the roads must be able to trust that the bridges are safe to drive on," the organisation's spokeswoman, Ingunn Handagard, told the Norwegian news agency NTB.
The Norwegian Public Roads Administration said it wanted an independent investigation into the collapse.
"It must be safe to drive on Norwegian roads. That is why it is important to get to the bottom of this case," said the administration's head, Ingrid Dahl Hovland.
Atle Formo, who lives by the Tretten bridge, said he heard "an intense crash".
"The whole house was shaking. I was rolling up the blinds in the bedroom and looked right at a bridge laying in the river," he told Norwegian broadcaster TV2.
A similar nearby bridge in Sjoa in the Gudbrand valley, also made of glued laminated timber, collapsed in 2016. The driver of a truck who was crossing the bridge at the time of the collapse was slightly injured.
Following that collapse, 11 similar bridges, including the one in Tretten, were temporarily closed by the government body that is in charge of Norway's infrastructure. The agency said in a report about the 2016 collapse that "the direct cause of the bridge collapse is a defective joint in the framework".
Norwegian transportation minister Jon-Ivar Nygård was due to visit the site of the latest bridge collapse later Monday.
- AP