This is the terrifying moment an overpass bridge collapsed onto a motorway, killing at least two people and injuring 15 others in Bangkok, Thailand.

A 500-metre-long section of the under-construction flyover plunged onto cars below when a steel girder crumpled to the ground during rush hour on Monday afternoon, July 10.

Project engineer Chatchai Prasert and worker Aran Sangkharak died at the scene, while at least 15 others were injured, authorities said. The wounded were taken to the Jularat Hospital and the Lat Krabang Hospital.

Power to the surrounding area in the Lat Krabang district of the capital was knocked out but was restored around midnight, local media reported.

Initial investigations found that a crane lifting a road segment had overturned and caused concrete and steel to plummet onto passing cars.

Bangkok Governor Chadchart Sittipunt said staff were barred from entering the area due to the unstable structure. Engineers and Public Works Department officials have been sent to survey the wreckage.

Rescue workers were combing through the debris until the evening to find more trapped bodies.

Governor Chadchart said: 'This should never have happened. Moving forward, we must thoroughly inspect projects and prioritise the safety of the people. There must be accountability as this was not an act of God.'

Narong Ruengsri, deputy permanent secretary of the Bangkok Metropolitan Administration, said today, July 11, that authorities were removing the debris to restore normal traffic flow. The clearing operations will take around a week, he said.

The flyover was part of an elevated road project seen to ease traffic congestion in the Thai capital.

Following the accident, construction work was indefinitely suspended as authorities determine the cause of the collapse.

Last May, a construction worker was killed when a concrete beam collapsed and struck cars along Rama II road in Bangkok.

Thailand has one of the world's worst road safety records. Ministers have set the goal of reducing fatalities from 32.7 deaths per 100,000 people to 12 per 100,000 people by the year 2027.

However, a lack of road safety education in schools along with notoriously easy driving tests, police failures to enforce road laws, and chronic under-investment in infrastructure, all appear to hamper the efforts.