Mr Abdullah said there might have been 'thousands of violations'
The main challenger in presidential elections held in Afghanistan last week, Abdullah Abdullah, has alleged widespread fraud.
Mr Abdullah said he had evidence that voting had been widely rigged in favour of incumbent President Hamid Karzai.
The allegations had been sent to the Electoral Complaints Commission (ECC) for investigation, he said.
A leading group of election observers has also said intimidation and voting fraud occurred in Thursday's poll.
The campaign teams for Mr Karzai and Mr Abdullah - a former foreign minister - each claim their candidate won an outright majority.
Preliminary results are due in the coming days, but the final result is not expected for several weeks.
Mr Abdullah said his team had been told that voter turnout had been significantly inflated in some areas where few votes were cast, with the extra ballots marked in favour of Mr Karzai.
"The initial reports we are receiving are alarming," Mr Abdullah was quoted as saying by Reuters news agency.
Was Afghanistan's election fair?
"There might have been thousands of violations throughout the country, no doubt about it."
Meanwhile, the ECC said it had received more than 200 complaints about the electoral process.
ECC spokesman Grant Kippen said the watchdog was aware of "significant complaints" of irregularities, including voter intimidation, violence, ballot box tampering and interference by some Afghan election officials.
But, he added, there were no specific charges against individual candidates such as Mr Karzai.
Afghan and Western officials have declared Thursday's poll a success, despite concerns about the turnout.
Richard Holbrooke, the US special envoy to Afghanistan, said allegations of fraud were to be expected.
"We have disputed elections in the United States," he said. "There may be some questions here. That wouldn't surprise me at all."
Mr Holbrooke said Washington would wait for rulings from both Afghanistan's Independent Election Commission and the ECC before commenting on the election's legitimacy.
The credibility of the vote may be brought into question by reports that turnout in some areas, such as the restive Helmand province, was as low as 5%, analysts say.
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