Poll stations attacked
"The turnout also needs to be at least 30 per cent, otherwise there will be problems - the position is not certain in any way."
Al Jazeera correspondent James Bays said that Karzai's main rival and former foreign minister, Dr Abdullah Abdullah, had insisted he was in the lead.
"Karzai and his team have reported they are leading, but Dr Abdullah has said, 'that's rubbish' and said he was well in front," Bays said.
"The election commission has said it may be unable to get full results until 3 September but unofficial results could start to come in within 24 hours.
"Dr Abdullah said he was very worried that, in some parts of the country, the turnover was extremely low because of the security situation.
"But the problem is at the street level - whether the people will accept the result."
Bombings and gun and rocket attacks struck across the country a day earlier on election day, leaving 26 Afghan security officials and civilians dead, the government said.
The Taliban had warned Afghans not to vote and threatened to cut off the fingers of those who voted and launch attacks on polling stations.
Nine civilians, nine policemen and eight soldiers were killed in 135 incidents, Afghan ministers said. Twenty-eight other soldiers were wounded and security forces thwarted five suicide attacks.
Amadshah Amadzai, a former Afghan prime minister and political analyst, told Al Jazeera he had decided to boycott the election weeks ahead of the poll.
"I told the people that this election will not be fair and that people will not attend to vote. My decision was very correct.
"The majority of the voters were not there. There were problems ... people just sat home and didn't go out for voting.
"People are completely disappointed in what has happened for the last seven, eight years.
"In the whole country these complaints are going on, because nothing has been done for the remote areas and south and western provinces of Afghanistan. Nothing has been done for the public."
Hamid Karzai, the Afghan president, said the election was conducted "successfully".
|Opinion polls suggest Hamid Karzai could face a run-off against Abdullah Abdullah [Reuters]
"The Afghan people defied rockets, bombs and intimidation and came out to vote, that is great," Karzai - the frontrunner of the 30 candidates - said from his fortified palace.
He said armed groups conducted attacks in 15 of Afghanistan's 34 provinces.
Afghanistan's election authority said it was launching an investigation into complaints of irregularities.
Abdullah Abdullah, released a statement detailing about 40 incidents of alleged irregularities, most claiming officials had pressured voters into choosing Karzai.
There were also claims voting centres did not have enough ballot papers.
About 17 million people were eligible to vote in presidential and provincial elections, with 6,500 polling stations open in 364 districts across the country.
Karzai needs to secure 50 per cent of the vote to win in the first round, but opinion polls before the election indicated that he was likely to face a run-off against Abdullah.