The blast came after a relatively calm weekend in which no major attacks were reported as Afghans voted for a new president and provincial councils.
The Taliban had threatened violence to disrupt Saturday's vote, and staged a series of high-profile assaults in the preceding weeks. But security forces tightened their grip and only sporadic attacks took place.
The two SUVs carrying the civilians hit the hidden explosives on a side road that was being used because authorities blocked the main road following a suicide bombing targeting a NATO convoy in Kandahar province, the local government spokesman said.
The Taliban claimed responsibility for the earlier attack on Monday but blamed international forces for the roadside bombing, saying the foreign forces were trying to hurt the reputation of the Islamic militant movement by making it look like the Taliban were killing civilians.
The suicide bomber was in a minivan when he detonated his explosives in front of the NATO convoy in the same district.
