According to reports from news agencies on the scene, hundreds more were injured after two spans - around 90m - of the concrete deck collapsed onto worker below at 8am in the morning local time (1am GMT).
Failure of a scaffolding falsework system supporting a section of the concrete bridge under construction has been given unofficially by police as the cause, according to Vietnamese online daily newspaper VNExpress. This was reported to have been poured the day before.
Heavy rain over the last few days in the area is thought to have weakened the falsework's temporary foundations and contributed to the collapse.
A huge operation is now underway on site to find and rescue the injured from beneath the concrete but is hampered by the scale of the debris. The government-backed rescue team is looking at ways to speed up this search and rescue operation but it is understood that there are concerns about the risk of further collapse.
So far there is no official detail of the cause but a full investigation into the collapse has been ordered by the Prime Minister Nguyen Tan Dung. Other eye witness reports have suggested that the concrete deck was being lifted and cut at the time.
The Japanese funded Can Tho Bridge crosses the Hau River a tributary of the Mekong River to link Vinh Long Province with Can Tho City in South Vietnam. The collapse site is some 170km south of Ho Chi Minh City.
Work started on the $345M bridge in late 2004 and was expect to be completed by 2008. At 2.75km long it will be the longest cable-stayed bridge in Southeast Asia. Around 250 people were working on the bridge when it collapsed.
The 26m wide reinforced concrete deck is supported by cable from two 165m high pylons was to carry four lanes of traffic giving 39m clearance to the river below.