According to Lal Mohammad Omarzai, governor of Shindand district in western Herat province, Mr. Manan was killed when he was walking from his home to a mosque for evening prayers.

The election is being seen as a key test of stability in Afghanistan, where violence is at its worst since the Taliban were ousted in 2001, before President Barack Obama conducts a review of his Afghan war strategy in December.

Poor security, particularly in Taliban strongholds in the south and east, already looms as the biggest challenge to the ballot, along with corruption and fraud.

About 2,500 candidates are contesting the ballot for 249 seats in the lower house of parliament.

Omarzai said two gunmen on a motorcycle rode up beside Manan and opened fire.