Presidential candidate Ashraf Ghani Ahmadzai (center) speaks during his last campaign rally in Kabul on Wednesday. The second round of Afghanistan's presidential election will take place on Saturday. Massoud Hossaini / Associated Press

Afghans head to the polls on Saturday for a second-round election to choose a successor to President Hamid Karzai, with threats of Taliban attacks and fraud looming over the country's first democratic transfer of power.

The vote pits former foreign minister Abdullah Abdullah against ex-World Bank economist Ashraf Ghani in a head-to-head contest to lead Afghanistan as US-led troops withdraw after 13 years of fighting Taliban insurgents.

April's first-round vote was hailed a success as turnout topped 50 percent and Islamist militants failed to launch any high-profile attacks on polling day.

But Saturday presents another major challenge in the prolonged election process, which began with campaigning in early February and will end when the final result is announced on July 22.

"There is concern that the enemy who failed in the first round will seek revenge, but we can assure you they will fail again," General Afzal Aman, head of Afghan army operations, told reporters.

"We have been conducting missions all over Afghanistan for election security for the past two months."

Ahead of the polls, the Taliban issued a warning to voters, saying that polling booths would be targeted by "nonstop" assaults.

"By holding elections, the Americans want to impose their stooges on the people," the insurgents, who were ousted from power by a US-led offensive in 2001, said on their website.

International fears have focused on the risk of tension between the candidates' supporters after the result, which may be contested if the count is close and serious fraud allegations are raised.

"No one should be complacent about what is at stake in the coming weeks," US Ambassador James Cunningham said in a statement. "We call on both candidates to direct their campaigns and supporters not to engage in fraud.

"It is our fervent hope that the two candidates, with the future of their country in their hands at this unprecedented time, will not seek a winner-take-all outcome."

The previous presidential vote in 2009 was riddled with fraud, damaging relations between the Afghan government and the US-led donor nations on which it relies for funding.

Abdullah eventually pulled out of that election, allowing Karzai to retain power, but this year neither candidate is likely to back down if the result is close.