An election worker count votes at a polling centre in a mosque in Kabul, Afghanistan, on Friday. (Rafiq Maqbool/Associated Press)

U.S. President Barack Obama hailed the Afghan presidential election as an "important step forward" as campaign teams for Afghan President Hamid Karzai and top challenger Abdullah Abdullah each claimed victory in Kabul on Friday.

Obama said it was the first democratic election run by Afghans in more than three decades. The 2004 election was run by the United Nations.

"This is an important step forward in the Afghan people's efforts to take control of their future even as violent extremists are trying to stand in their way," Obama said at the White House, a day after the election.

Obama said regardless who wins, the U.S. will continue to work with Afghans to stabilize the country.

"Our goal is clear: to disrupt, dismantle and defeat al-Qaeda and their extremist allies," he said. "That goal will be achieved and our troops will be able to come home as Afghans continue to strengthen their own capacity and take responsibility for their own future."

Millions of Afghans braved militant attacks to cast ballots Thursday, although turnout was weaker than the previous vote in 2004 because of fear, disenchantment and election-related violence, which killed 26 people.

Campaign teams posted results, based on reports from their polling site observers. Abdullah's unofficial returns showed him beating Karzai handily — but did not include any numbers from the south and east, where Karzai was expected to win large majorities.

Across town, Karzai's campaign team said the president had garnered more than 50 per cent of the vote, a result that would negate the need for a two-man runoff.

However, the country's Independent Election Commission said it is too early for any campaign to claim itself the winner.

Preliminary results won't be made public before Saturday and final official results are not expected until early September.

Grant Kippen, the Canadian chairman of the Afghan Electoral Complaints Commission, said Friday it's too early to say how many complaints were filed across the country.

"I think there were a lot of doom and gloom scenarios being discussed before election day and I think it went quite well," Kippen told The Canadian Press.

"This is an emerging democracy. Yes, there's going to be some problems along the way."

So far, most complaints relate to process issues such as polling centres opening late, but there have also been allegations of fraud and intimidation at some centres.

"Right now everybody requires a little patience as this process works its way through," Kippen said.

With files from The Associated Press