Selangor Fire and Rescue (Operations) assistant director Mohd Sani Harul said the bus was heading to Kuala Lumpur when the accident happened at 4.50am.
It was believed to have hit the road divider and turned on it side, crossing over to the opposite side of the highway.
“We immediately deployed 21 personnel from the Kuala Kubu Bharu and Rawang fire and rescue stations to the scene,” he said.
He said all the victims were sent to the Sungai Buloh Hospital for treatment.
Hulu Selangor district police chief Supt Lim Bak Phai said initial investigations revealed that four passengers sustained serious head injuries while the bus driver was injured in the face.
“The actual number of bus passengers travelling from Sungai Petani to Kuala Lumpur and onwards to Temerloh has yet to be ascertained. A further probe is being carried out,” he said.
Lim said the police would conduct urine and blood screening on the bus driver and his assistant to facilitate investigations.