This is the terrifying moment a bus carrying 20 passengers including two tourists crashed when its driver suffered a heart attack at the wheel.

Wichai Chuensanguan, 73, had picked up tourists from the Erawan Falls and was headed to Kanchanaburi province in Thailand when he fell unconscious while driving on September 14.

As he was turning a curve, he suddenly fainted, leaving no one to steer the bus properly.

Dashcam footage shows how the vehicle moved on the wrong lane before hitting a pick-up truck as it swerved off the road.

The Kanchanaburi Tourist Police arrived at the scene and found the truck damaged on its right side, while the bus was tangled in the undergrowth around 10 metres from the curve.

Wichai was still strapped in the driver's seat, unconscious with no pulse. Medics performed CPR on him, but he could not be revived and was pronounced dead at the scene.

Officers said at least seven people were injured, including two foreign tourists. They were taken to the Phahon Phon Phayuhasena Hospital for treatment.

Authorities believe Wichai had tried to park the bus on the roadside when he felt something wrong, but lost consciousness before he could pull over safely.