Afghan electoral workers sort ballot boxes at a counting centre in Kabul April 10, 2014.
Afghanistan's Independent Election Complaints Commission said on Sunday more incidents of serious fraud were reported in an April presidential election than in the previous one in 2009, when more than a million suspect votes were thrown out.
The complaints body hinted it might need more time than expected to investigate all of the complaints though the volume would not affect the overall schedule for electing a leader. Final results are due on May 14.
"There is a possibility, in order to review the high number of complaints accurately, that we may expand the time frame for reviewing complaints in provinces for some days," said IECC spokesman, Nader Mohseni.
Afghanistan's allies praised the April 5 vote as a success because of the high turnout, estimated at 60 percent of 12 million eligible votes, and the failure of Taliban militants to stage high-profile attacks on the day.
But evidence of widespread fraud could undermine the legitimacy of an election meant to usher in Afghanistan's first democratic transfer of power, as incumbent Hamid Karzai prepares to step down after more than 12 years in power, and as Western forces prepare to leave.
The three frontrunners have all complained of fraud.
To win, a candidate must secure more than 50 percent of valid ballots. Failing that, the top two candidates go into a run-off.
Partial results from a sample of ballots were expected on Saturday but they have been delayed to later on Sunday.
The IECC has recorded a total 870 incidents of fraud classed as "Priority A", complaints considered serious enough to affect the outcome of the election, higher than the 815 incidents recorded in 2009.
Video clips of polling station workers and other people stuffing ballot boxes are circulating on the internet, but it remains unclear whether fraudulent votes might have benefited any one candidate over another.
In 2009, ballot-box stuffing was the most common type of fraud. The complaints commission has yet to disclose which type of suspected fraud was most prevalent this time.
Overall, the IECC has recorded a total 3,724 complaints, exceeding the total of 3,072 in 2009.
The number could rise as complaints reported in the provinces reach Kabul.
Urban participation in the election was unexpectedly high, but it is unclear to what extent rural voters were deterred by the Taliban, who condemned the vote, and what role state officials, including police, had in encouraging people to back a particular candidate.