Afghanistan's first democratic election has been thrown into confusion after it was announced that most presidential candidates were boycotting it.
Afghan women line up to vote at a polling station in Kabul
The move follows claims of widespread voting irregularities.
The boycott was agreed by 15 candidates opposed to the favourite, the interim President Hamid Karzai, reports say.
A spokesman for the joint UN-Afghan electoral commission said the vote would continue as it had been generally "safe and orderly".
BBC News Online's Sanjoy Majumder in Kabul says it will be a major setback for poll organisers if the credibility of the election is undermined.
The vote has been widely seen as a chance for Mr Karzai to extend his authority beyond the capital, Kabul.
He has led the country since the fall of the hard-line Islamic Taleban nearly three years ago.
Despite the wrangling among the candidates, BBC correspondents around Afghanistan have reported great levels of enthusiasm among ordinary people for the democratic process.
'Call it off'
The boycott has for now overshadowed fears that Taleban militants might disrupt the vote.
"Today's election is not a legitimate election," presidential candidate Abdul Satar Sirat said after hosting a meeting in which it is reported that 15 candidates signed up to the boycott.
"It should be stopped and we don't recognise the results."
The allegations of voter fraud arose after complaints that the indelible ink used to mark voters' fingers can be washed off.
The only woman candidate, Masooda Jalal, told the Associated Press: "The ink that is being used can be rubbed off in a minute. Voters can vote 10 times."
Earlier on Saturday polling was briefly suspended in some areas of the capital, Kabul, and the northern city of Mazar-e-Sharif while the complaints were investigated.
But our correspondent says the extent of potential fraud remains to be seen, and the row has provided a convenient rallying point for a previously disunited opposition.
A senior United Nations official helping supervise the vote said that "overall it has been safe and
The indelible ink must be applied to the cuticle of the voter's fingernail
"The vote will continue because halting the vote at this
stage is unjustified and would deny these people their
right to vote," Ray Kennedy said.
On hearing reports of the boycott, some voters in Kabul urged candidates not to be hasty.
"This is Afghanistan's first election. There are bound to be problems. Give it a chance," one man, Dost Mohammed, told BBC News Online.
But others were more sceptical.
"I believe those who have boycotted the elections are right. I saw myself that I could easily rub the ink from our fingers," said Ibadullah, a student.
Apart from Mr Karzai, only two other candidates have not signed up to the boycott, reports say. They both stood down in his favour on the last day of the campaign.
It is not clear how the election organisers will respond to the boycott. Earlier they had insisted that voting would continue.
Eighteen presidential candidates
Over 10.5m Afghans registered to vote
41.3% of voters are women
Men and women to vote in separate polling stations
More than 25,000 polling stations
About 5,000 counting centres
More than 130,000 polling officials
About 740,000 Afghan refugees from Pakistan expected to vote
About 600,000 Afghans in Iran eligible to vote
Farook Wardak, head of the Joint Election Management Board, said marker pens were being used in some cases instead of the indelible ink, while in other cases, the correct ink was being applied to the wrong part of the finger.
"Where the correct ink is applied it stays on," he told BBC News Online.
President Karzai voted early in Kabul, saying it was a great day for the Afghan people.
He is widely tipped to win, although Uzbek General Abdul Rashid Dostum and Tajik former education minister Yunus Qanuni have fought high-profile campaigns.
Correspondents say much will depend on how the country's various power brokers react to the result and how far the victor is prepared to challenge the political status quo in a country sometimes described as a series of mini-fiefdoms.
Security has been the leading concern in the run-up to the election, with up to 100,000 Afghan and international security personnel on high alert.
The first vote was cast by an Afghan refugee in neighbouring Pakistan, where voting opened slightly earlier.
Hamid Karzai cast his vote early
"I am very happy," said 19-year-old Moqadasa Sidiqi, after she voted in the Pakistani capital, Islamabad.
Voting was scheduled to finish at 1600 local time (1130GMT), but was extended in some areas as many people still wished to vote.
Ballot boxes will then be sealed and transported to eight regional counting centres.
Initial results are expected in the coming days but it may take a couple of weeks for all the votes to be counted.