Angry Afghans blocked the main road out of Kandahar in protest at the killing of four civilians after foreign forces reportedly opened fire on a local bus.
Eighteen other passengers were wounded in the incident, some of them seriously.
Witnesses said an American military convoy fired on the vehicle when it drew up close in the west of Kandahar city. NATO has launched an investigation.
The issue of civilian casualties caused by international troops who are meant to protect them is undermining support for their presence in the country.
It has also caused a rift between the Afghan president and the West.
Last year more than two and half thousand non combatants were killed in Afghanistan, making it the deadliest 12 months of the eight year war. NATO’s top commander in Afghanistan has issued strict orders to his troops to try to reduce the number of fatalities.