The night bus, Swebus Express, was en route from Copenhagen to Oslo, according to Ingvar Ryggesjö, press officer for Swebus.
The accident occurred just before 4 am on a temporary road that is currently under construction. The area was under heavy fog.
Although it was moving slowly, the bus lost control in the downpouring rain and fog, drove into a ditch and tipped over, according to Swebus.
A total of nine people were injured, eight passengers and the driver. They were taken to the hospital in Uddevalla by ambulance. At least one person was in critical condition, reported SOS Alarm emergency responders.
An additional 11 passengers received treatment. There were a total of 44 individuals, including the driver, on the bus. All traffic on the E6 was blocked due to all of the emergency vehicles.