All 15 candidates opposed to interim President Hamid Karzai withdrew in the middle of the voting Saturday, after at least some voters were able to wipe away ink from their fingers, news agencies reported. The ink is supposed to remain visible to prevent multiple voting.
A member of the Joint Electoral Management Body, Joseph Brewin, said the ink is only one of the safeguards, and its absence does not invalidate the whole voting process. Vote counting is likely to start on Monday, after all ballot boxes have been brought in from outlying areas.
Electoral officials have pledged to thoroughly investigate the claims of irregularities.
Karzai has praised the first-ever presidential election, saying Afghans had participated "massively."
One of the largest group of election observers also said the voting went off relatively well. The independent Free and Fair Elections Foundation said a "fairly democratic environment had been observed in the overall majority of polling centers".