KABUL - Leading candidates in Afghanistan's presidential election voiced concern Sunday that voting was tainted by fraud, a day after millions defied Taliban threats and turned out to choose a successor to President Hamid Karzai.
World leaders praised the courage of voters, who came out in force to cast their ballots despite bad weather and a violent campaign of intimidation, and urged patience during the long counting process.
Ahead of the poll there were fears that a repeat of the massive fraud which blighted Karzai's re-election in 2009 would undermine the winner's legitimacy at a testing time for the war-torn country.
There were reports of polling stations in numerous parts of the country running out of ballot papers, leaving some people unable to take part.
More than 1,200 complaints had been received by the Election Complaints Commission (ECC) by Sunday morning, spokesman Nadir Mohsini said, and the number was increasing.
"Complaints include late opening of polling centres, shortage of ballot papers, encouraging of voters to vote for certain candidates and mistreatment of some election officials," Mohsini said.
But in a promising sign for the stability of the process, two of the frontrunners to succeed Karzai, who is stepping down after serving a maximum two terms, said they would abide by the ECC's rulings.
Zalmai Rassoul, who was seen before the vote as Karzai's preferred choice, told a press conference on Sunday that he had made complaints to the ECC but refused to give details.
He said he was confident it would address the concerns properly but warned: "Any president elected with fraud will not be accepted by Afghanistan."
- 'Reports of serious fraud' -
Former World Bank economist Ashraf Ghani, who claimed to be in the lead in preliminary results, voiced similar views.
"There are reports of serious fraud in several locations but all is documented and will be passed on to ECC for investigation," he said on his official Twitter account.
Whoever emerges victorious from the lengthy vote-counting and appeals process faces the prospect of fighting the Taliban without the help of US-led combat troops.
There were fears that a disputed result could spark bitterness and recrimination and put the new president in a weak position.
The third leading candidate, Abdullah Abdullah, runner up in 2009's acrimonious poll, said his team had also filed complaints.
"Unfortunately, hundreds of our countrymen were deprived of their rights due to shortage of ballots. The process was not free of flaws," he said. "We have registered complaints about fraud."
An estimated seven million people voted, according to the Independent Election Commission (IEC), more than 50 percent of the electorate -- a huge increase on 2009 when only around a third of those eligible cast ballots.
Preliminary results are due on April 24 and if no candidate secures more than 50 percent of the vote, a runoff is planned for late May.
- Bomb destroys ballots -
A roadside bomb hit a truck carrying full ballot boxes in the northern province of Kunduz on Sunday, killing three people, officials said. Eight boxes of votes were also destroyed in the blast.
The Taliban had urged their fighters to target polling staff, voters and security forces. But no major attacks were reported during the day, although sporadic violence caused a number of casualties.
Emanuele Nannini, programme coordinator for International NGO Emergency which runs three hospitals in Afghanistan, said they treated around 30 people on Saturday, which he said was an unusually high number.
The day before the poll, Associated Press photographer Anja Niedringhaus was shot dead by a police commander in the eastern province of Khost.
US President Barack Obama congratulated Afghanistan on the largely peaceful ballot -- its first democratic transfer of power -- and said it was "critical" to its future and securing continued international aid.
"This is their moment. The Afghan people secured this election. They ran this election, and most importantly, they voted in this election," US Secretary of State John Kerry said in a statement.
The United Nations Security Council also congratulated Afghanistan and urged the candidates and their supporters to be patient and "respect the electoral institutions and processes".
The election will end 13 years of rule by Karzai, who has held power since the Taliban were ousted in 2001.
Afghans have taken over responsibility for security from US-led forces, and this year the last of the NATO coalition's 51,000 combat troops will pull out.