Abdul Rahman Zhuwandai, a spokesman for Farah province, said the civilians were riding in a bus that hit the bomb in Pusht Rod district.
At least 10 people were wounded in the explosion in what is a relatively peaceful part of the country.
Afghan President Hamid Karzai condemned the attack, saying, "Our people want to live in joy and happiness."
Separately, two service members with the U.S-led military coalition were killed Friday by a roadside bomb in eastern Afghanistan, the coalition said. Their nationalities were not released.
So far this year, 377 U.S. and other foreign troops have been killed in Afghanistan