KABUL - Voter registration cards are for sale by the handful on the streets of Afghan cities and villages.

Somebody even registered U.S. pop star Britney Spears to vote in Thursday's presidential election -- copies of her card were widely emailed and, for a while, pinned up in a Kabul hotel bar.

One Afghan man in a village not far from Kabul had a sackful of cards buried by a stream at the back of his house, for sale to anyone who asked -- but he'd prefer if they were used to vote for President Hamid Karzai.

The question is not whether fraud will be perpetrated in Thursday's election, already described as imperfect by the United Nations and many Western observers, but whether it will be of sufficient magnitude to influence the outcome of the vote.

What would be an acceptable outcome is difficult to gauge, with the danger of widespread civil unrest a major concern if the result of the poll, Afghanistan's second since the Taliban were toppled in 2001, is disputed.

The acceptance of the results from the top candidates and their supporters is absolutely vital, said Dan McNorton, a spokesman for the United Nations mission in Afghanistan.

With security and threats of violence already a major factor in the election, uncertainty over the result of the poll could fuel further unrest by allowing insurgents to exploit the vacuum.

Perceptions of illegitimate election results will benefit the Taliban, policy think tank The International Council on Security and Development (ICOS) said in a recent election report.

This will increase if no one wins an outright majority on Thursday and the vote goes to a second round in October, it said.

Recent attention has focussed on voter registration cards, genuine or fake, with suspiciously large numbers of women registered in some areas. But election officials and monitors say other measures can still block fraud, if applied correctly.

Afghans will have their fingers marked with indelible ink before casting their ballots, meaning they will only be able to vote once no matter how many registration cards they might have.

The concern is not at the front end of the process but at the back end. It's not just a question of false registration cards, you need complicity at the polling station to use the cards fraudulently, ICOS President Norine MacDonald told Reuters.

Afghanistan has about 15 million eligible voters in a population of about 33 million, according to the government-appointed Independent Election Commission, with about 4.5 million new voters registered this year.

Glenn Cowan, a governance expert and co-founder of Democracy International, thinks it will be impossible for international observers to quantify the amount of fraud.

But he doubts that what he described as retail corruption -- attempting to influence the outcome one vote at a time -- would have much of an impact.

If you're going to move 2 or 3 million votes in order to influence an election, that is a very, very difficult thing to do, Cowan told Reuters.

At that level, Cowan is more concerned with the coercion of individual voters by local warlords and chieftains, as well as the Taliban's threats to disrupt the poll.
Because of security issues this is not an entirely open and free choice, Cowan said.

The likelihood of unrest will increase if the vote goes to a second round or if Karzai's main challenger, former foreign minister Abdullah Abdullah and his ethnic Tajik supporters, suspect Karzai, a Pashtun, has won unfairly.

The winner of either the first or the second round could have his legitimacy questioned by allegations of fraud which could destabilise the country and raise the possibility of political violence and civil unrest, MacDonald said.