The outcry over alleged vote fraud in Afghanistan's election has escalated, with President Hamid Karzai's chief opponent charging that turnout figures were padded and the chief fraud investigator saying some of the allegations were serious enough to influence the outcome if true.

The controversy threatens to discredit an election that the Obama administration considers a key step in a new strategy to turn back the Taliban insurgency. It could also delay formation of a new government and fuel growing doubts in the United States about whether its worth continuing to fight the war in Afghanistan.

Millions of Afghans voted Thursday 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 support for Karzai is strong.

Final certified results will not come until next month although partial preliminary figures are expected Tuesday. If none of the 36 candidates wins a majority, the top two finishers will face a runoff in October.

Karzai's top challenger, former Foreign Minister Abdullah Abdullah, widened allegations of fraud against Karzai and his government Sunday, saying ballots marked for the incumbent were coming in from volatile southern districts where no vote was held, and that turnout was being reported as 40 percent in areas where only 10 percent of voters cast ballots.

"This is a sign or evidence of widespread rigging," said Abdullah, who draws his strength from the Tajik minority in the north.

Abdullah said a border security commander in the Spin Boldak district of southern Kandahar province, Gen. Abdul Raziq, used his house as a polling station and stuffed the ballot box for Karzai. Other polling sites were in border police posts that Raziq controls, Abdullah said.

Another presidential candidate has displayed mangled ballots that he said were cast for him in Spin Boldak and then thrown out by election workers.

Raziq denied the charges, saying that everyone in Spin Boldak voted in the appointed polling centers, which were schools and mosques. He said he and his border police were busy maintaining security and did nothing to tamper with the process.

"They are just spreading propaganda, the people who are saying there was fraud," Raziq said. "If there is any proof of it, please show me."

Abdullah said he hoped fraud would be prevented through legal appeals with the electoral complaints commission. But he also said he had no faith in the chief of the Afghan Independent Election Commission, a Karzai appointee.

The Canadian head of the electoral complaints commission, Grant Kippen, said his group had received 225 complaints since polls opened Thursday, including 35 allegations that are "material to the election results," which means if true they could influence the outcome.

The commission must complete its investigation into major complaints before a winner can be certified or a runoff announced.

The top Afghan monitoring group has said there were widespread problems with supposedly independent election officials at polling stations trying to influence how people voted. That group, the Free and Fair Elections Foundation of Afghanistan, also catalogued violations such as people using multiple voter cards so they could vote more than once, and underage voting.

U.S. and NATO officials, who were hoping for a successful Afghan election, have been quick to point out that it's too early to determine if the fraud allegations were true and whether they were extensive enough to undermine the credibility of the balloting.

The Obama administration is anxious for a winner with a clear mandate to confront the Taliban, combat corruption, curb drug trafficking and rebuild the economy. Without a credible election, all that is at risk.

The U.S. special envoy to Afghanistan said allegations of vote rigging and fraud are to be expected, but observers should wait for the official complaints process to run its course before judging the vote's legitimacy.

"We have disputed elections in the United States. There may be some questions here. That wouldn't surprise me at all. I expect it," Richard Holbrooke told AP Television News in the western city of Herat. "But let's not get out ahead of the situation."