The explosion occurred near Grishk City in southern Helmand province on the main highway in the area, said Patrick Fruchet, spokesman for the UN Mine Action Centre for Afghanistan.
Two of the deminers were killed and five others wounded. Three of the wounded were in critical condition, Fruchet said.
The seven were employed by a local demining organisation - the Mine Detection and Dog Centre - and were working on a project funded by the Japanese government, he said.
Fruchet had earlier said that the men were working on a project funded by the US Agency for International Development (USAID), but he later clarified that it was in an adjacent area.
Two mine-detecting dogs that were in the car were also killed, Fruchet said.
Shohab Hakimi, head of the Mine Detection and Dog Centre, said the bomb was hidden on the back of a bicycle that had been left on the bridge and was detonated by remote control.
Hundreds of deminers are working across Afghanistan, slowly clearing hundreds of thousands of mines left over from a quarter century of war.
Some 200,000 people in Afghanistan are thought to have died or been injured by mines and other unexploded ordinance in the past 25 years.