Another 10 people were wounded in the incident on Thursday in Tarin Kot, capital of Uruzgan province, said Abdullah Jan, a spokesman for the provincial governor.
"It was a suicide attack and we are not sure how many of the 10 people killed in the attack are civilians and how many are policemen," he said.
The attack happened in a bazaar a few hundred metres from the headquarters of the provincial government. Some of the wounded were taken by helicopter to hospital in the neighbouring Kandahar province, the spokesman said.
Taliban insurgents are suspected to have carried out a spate of human bombings in recent weeks.
Also on Thursday, Afghan security forces blew up a car packed with explosives they believe was intended for an attack on US-led forces in the southern town of Spin Boldak, which is also in Kandahar, on the Pakistani border, police said.
On Monday, an attacker drove a car bomb into a convoy of foreign troops in the southern Afghan city of Kandahar, wounding a US soldier and an Afghan woman and a child.