The incident took place at around 4am, when a tour bus from Bueng Kan's Phon Charoen district went off the curve on Highway 304 during the journey to Rayong province.
The vehicle overturned and was completely damaged. Local policemen and rescuers are rushing to transport the injured to Nadi Hospital and Kabinburi Hospital for medical treatment.
The road is being cleared to facilitate traffic, while the investigation is underway to determine the cause of the accident.
According to officials, the accident involved the third of three tour buses carrying 47 members of the village waste bank in Bueng Kan's Phon Charoen district and two drivers.
A passenger on the overturned bus said that he was forced to wake up due to the accident, causing many people in the vehicle to be thrown in different directions. All passengers are neighbours who live close by each other, he said.
A trailer driver Chayapol Sophakul (45) explained that he followed behind the bus until the accident occurred. He got off the trailer and found that more than 40 passengers had been injured, mostly inside the vehicle.
Chayapol confirmed that he had contacted the police to survey the scene after the incident.
Meanwhile, Na Di district chief Somjai Phutthasena said officials had transported around 20 injured passengers to hospitals and feared that other passengers might not survive. Officials are accelerating their search for passengers, he said.
He noted that road accidents often occurred on the way down from Khao San Pu Tone mountain, causing many injuries and fatalities. He expects the accident came due to a malfunctioning brake system.