The policemen were attacked late on Saturday on a road connecting the main southern city of Kandahar to the capital, Kabul.
Muhammad Nabi Allahyar, highway police commander, said: "The fighting lasted for more than one-and-a-half hours. It was fierce fighting."
Allahyar blamed the attack in Zabul province on remnants of the Taliban, which was in power from 1996 until 2001, when it was removed by a US-led invasion of Afghanistan.
Some of the 10 to 15 attackers were also wounded in the gunfight but their comrades took them away, he said.
The attack occurred two days before the first sitting of the war-ravaged country's legislature due on Monday after more than 30 years of conflict.
Heavy security has been put in place for the opening ceremony to be attended by Hamid Karzai, the president, and foreign dignitaries including Dick Cheney, the US vice-president.
Unrelenting attacks on targets linked to the US-backed government have killed more than 1500 people this year.