President Hamid Karzai's leading challenger accused him of using the Afghan state to "rig" this week's election and detailed allegations of cheating by government officials in an interview Saturday with The Associated Press.

Abdullah Abdullah, once Karzai's foreign minister, said he was in contact with other campaigns to explore the possibility of a coalition candidacy in case none of the 36 candidates won enough votes in last Thursday's ballot to avoid a runoff, probably in October.

The accusations, which Karzai's spokesman denied, are the most direct Abdullah has made against the incumbent in a contest that likely has weeks to go before a winner is proclaimed. Both Abdullah and Karzai claim they are in the lead based on reports from campaign poll watchers monitoring the count.

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On the left, Afghan incumbent President Hamid Karzai holds up his finger, inked purple after casting his election ballot, as he meets with the media in Kabul on Thursday. On the right is his chief rival, Abdullah Abdullah, who on Saturday accused him of rigging the vote.

Officials of Abdullah's campaign have alleged fraud in several southern provinces where the insurgency is strongest and Karzai had been expected to do well.

"He uses the state apparatus in order to rig an election," Abdullah said in the interview. "That is something which is not expected."

Abdullah said it "doesn't make the slightest difference" whether Karzai or his supporters ordered the alleged fraud.

"All this happens under his eyes and under his leadership," Abdullah said. "This is under his leadership that all these things are happening, and all those people which are responsible for this fraud in parts of the country are appointed by him. And I'm sure he has all those reports, so he knows all of this. This should have been stopped and could have been stopped by him."

'This should have been stopped and could have been stopped by him'—Abdullah Abdullah on allegations of vote-rigging by Hamid Karzai

If Abdullah supporters believe the election was stolen, it could lead to the type of street violence that marred Iran's presidential election in June. Abdullah has called for calm and says grievances should be resolved through the country's Electoral Complaints Commission.

Abdullah said during the interview that government officials in Kandahar and Ghazni provinces, including a provincial police chief and a No. 2 provincial election official, stuffed ballot boxes in Karzai's favour in six districts. He also said his monitors were prevented from entering several voting sites.

Karzai's campaign spokesman Waheed Omar dismissed Abdullah's allegations and claimed the president's camp had submitted reports of fraud allegedly committed by Abdullah's followers to the election complaint commission.

"These are not new allegations. These were made even before the election took place," Omar said. "We have documented violations that were made by Abdullah's campaign team. But we believe our job is to report to the elections complaint commission … We do not want to make a media propaganda campaign out of the violations we have documented."

Omar said losing candidates often claim fraud to "try to justify their loss."

Millions of Afghans voted in the country's second-ever direct presidential election, although Taliban threats and attacks appeared to hold down the turnout, especially in the south where Karzai was expected to run strongly among his fellow Pashtuns. Election observers have said the voting process was mostly credible, but are cataloguing instances of fraud and violence.

Preliminary results will not be released until Tuesday and final certified results won't come until next month. If neither Karzai nor Abdullah gets 50 per cent of the vote among a field of about three dozen candidates, then the two will go to a run-off.

Election day violence

Meanwhile, Taliban militants carried out dozens of attacks on election day, violence that killed 26 Afghan civilians and security forces. Abdullah said he had expected better security and more competence from the election authorities.

On voting day itself, Taliban militants cut off the ink-stained fingers of two voters in Kandahar province shortly after casting ballots.

Also Saturday, the U.S. command reported that an American service member died of a non-combat injury in eastern Afghanistan. No further details were released. The death brought to 35 the number of U.S. service members to die in Afghanistan this month.