Mr Karzai appears to be in the lead, but many observers are focused on whether Afghans will be able to carry-out a credible election.

Afghanistan's southern Zabul province is regularly hit by Taliban attacks, but residents in the main city say they are planning to participate in Thursday's poll - despite threats from militants.

It remains unclear how many registered voters will participate. Taliban forces are now believed to exert influence over as much as half of the country, but election officials have predicted that perhaps 90 percent of polling stations will be open.

Thousands of additional US and NATO troops are in Afghanistan to help secure the polls.