Afghan election authorities rejected fraud allegations against top officials on Monday after front-running presidential candidate Abdullah Abdullah unleashed accusations that could threaten a smooth transition of power.

Abdullah's fraud claims put him in direct conflict with the Independent Election Commission, raising fears of political instability as the bulk of US-led troops withdraw from Afghanistan by the end of the year.

Abdullah demanded the sacking of Zia-ul-Haq Amarkhail, head of the IEC secretariat, over Amarkhail's alleged attempt to remove unused ballots from the IEC headquarters in Kabul on polling day.

He also said the IEC's turnout figure of 7 million voters in Saturday's runoff election was probably false.

But IEC chairman Ahmad Yousuf Nuristani rejected the accusations against Amarkhail, saying the turnout figure was an early estimate that might be adjusted.

"I strongly reject these allegations," Nuristani said, adding that Amarkhail was stopped by police when he was overseeing the delivery of extra ballot papers to polling stations that had run out.

"It was a misunderstanding between police and our staff," Nuristani said. "We do not want a crisis for the people of Afghanistan - they are tired of crises."

Ghani accused

Abdullah said he believes fraud denied him victory in the 2009 presidential race and has often said that only widespread ballot-rigging could stop him from winning this time.

He told reporters on Monday that he "always asked supporters to show restraint" but that the problems could turn into a "crisis".

He also said his opponent, Ashraf Ghani, was "absolutely" involved in fraud.

Ghani's campaign team responded by saying it had faith in the election authorities and their ability to ensure a fair result.

"We believe (Abdullah's) comments are in contradiction to the Afghan Constitution, election laws and the democratic process," a Ghani spokesman said.

Counting the votes will take weeks. The preliminary result is due on July 2, before the official complaints period begins, and the final result is scheduled for July 22.

The two candidates went through to the runoff after finishing at the top of an eight-candidate field election on April 5, when Abdullah secured 45 percent and Ghani scored 31.6 percent.

President Hamid Karzai, who has ruled since the fall of the Taliban in 2001, was constitutionally barred from standing for a third term.

Voter intimidation

Saturday's election was hailed as a success despite more than 50 people being killed in separate Taliban strikes on polling day.

Eleven voters in the western province of Herat had their fingers - which were dipped in ink to register their ballot - cut off by insurgents.

All voters in Afghanistan had their fingers marked with ink after voting to prevent them from casting more than one ballot, but the ink also identified those who participated in the election in defiance of Taliban threats.

Police say they hunted down and killed two Taliban insurgents responsible for the attacks. "Insurgent commander Mullah Shir Agha and one of his officers were killed in a police operation yesterday in Herat," a statement from the Interior Ministry said.

A successful election is a key test of the 13-year international military and aid effort to develop Afghanistan since the fall of the austere Taliban regime in 2001.

All NATO combat forces are due to withdraw from the country by December, though 10,000 US troops will remain into next year.