The bombing, the second suicide attack in Kandahar in as many days, took place on the main road heading west to the troubled province of Helmand.
At least six of the dead were civilians including women and children and at least three were security guards.
More than 3,700 people have been killed in Afghan violence this year.
Reports say the attacker was driving a car and targeted a four-vehicle convoy of a security firm which was preparing to set out from an assembly point.
Many of the casualties were bystanders or passengers in a nearby minibus, the BBC's Charles Haviland reports from Kabul.
On Friday, a bomber killed a district governor in Kandahar as well as his three young children.
Kandahar is Afghanistan's main southern city and the headquarters for the southern division of the Nato-led intervention force.
It is also the traditional heartland of the Taleban rebels, our correspondent adds.
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