The car had left Kandahar city, the capital of Kandahar province, on the way to Panjwavi province, which has a large Taliban presence.
Forced from power
"Seven civilians were martyred including three children, two women and two men," Mohammad Shah Khan, the deputy provincial police chief, told the Agence France Presse newsagency.
Khan said that "enemies of Afghanistan", a term used to describe Taliban, undertook the attack.
The Taliban has been fighting Afghan and international forces since the US military forced them from power after invading the country in 2001.
The UN has said that just over 1,000 civilians were killed in fighting in the first six months of 2009 - a 24 per cent increase on the same period in 2008.
Air raids by international forces have made up a significant number of civilian deaths.