Afghan election officials say they have received some 2,500 written complaints of fraud and irregularities following Saturday's parliamentary elections.
Hundreds more complaints have been received over the telephone, they say.
The Electoral Complaints Commission will investigate the allegations of fraud before results are announced. Final results are due in late October.
Last year's presidential poll - won by President Hamid Karzai - was also marred by widespread fraud.
The BBC's Dawood Azami in Kabul says that this year's complaints include allegations of multiple voting, using fake identity cards and stealing ballot boxes.
There are also concerns that supposedly indelible ink - to ensure that no one votes more than once - may have washed off in many cases. Voter intimidation by powerful candidates is among other major complaints.
More than 2,500 candidates stood on Saturday for the 249 seats in the Wolesi Jirga, the lower house of parliament.
UN Secretary General Ban Ki-moon said Afghan voters had shown "courage and determination" to cast their ballots.
Thousands of fake voter registration cards were seized days ahead of the election.
More than 5,000 polling stations opened on schedule on Saturday. About 1,000 did not for security reasons.
On the day, more than 400 violent incidents were recorded, most blamed on Taliban-led militants who were opposed to the poll.
Violence and allegations of massive fraud also dogged last year's vote.
Election officials called for a re-poll in the end, but President Karzai's main contender, Abdullah Abdullah, pulled out of the race questioning the credibility of a second vote.
The election watchdog said the 2,500 complaints it had received so far this year did not include incidents linked to security or threats by the Taliban.