KABUL, Afghanistan (AP) _ Poor-quality ink that can easily be rubbed off may be allowing some voters in Afghanistan's first direct presidential election to cast more than one ballot, causing a headache for election workers and an uproar among candidates who say it will lead to cheating.

The problem surfaced at polling stations in Kabul and the northern city of Mazar-e-Sharif, but could be a problem across this nation of 25 million people, who have almost no experience in organizing an election.

``It is a mistake. I hope it won't be an issue in all the centers, but we are investigating,'' said electoral commission spokesman Aykut Tavsel.

About 10.5 million registration cards were handed out ahead of the election, a staggering number that even U.N. and Afghan officials acknowledge was inflated by widespread double registration. Human rights groups say some people obtained four or five voter cards, thinking they would be able to use them to receive humanitarian aid.

Vote organizers had argued that the indelible ink would prevent people from voting twice, even if they had more than one card.

But voters in many parts of Kabul showed reporters how easy it was to rub off the ink. Some minor candidates have complained that it was an attempt to cheat.

``My own family and our neighbors are back from the polling stations, and very easily they can erase the ink,'' said Hafiz Mansoor, a Tajik newspaper editor who is one of 15 people challenging interim leader Hamid Karzai. ``This is a trick that is designed to clear the way for cheating!''

In the northern city of Mazar-e-Sharif, one voter said he had seen people go back to vote more than once.

``This ink only lasts for 10 minutes. Some people are voting 10 times! This is cheating,'' said Aziz Ullah Rozi.