Long queues formed outside polling stations in cities across the country, despite cold, wet weather, as voters cast their ballots at around 6,000 centres under tight security.

While voting in urban areas appeared brisk, it was not clear what turnout would be like in rural districts.

The Taliban have rejected the election as a foreign plot and urged their fighters to attack polling staff, voters and security forces, but only one incident was reported in the first few hours of voting.

A blast killed one person and wounded two others at a school being used as a polling station in Logar province, south of Kabul, district chief Abdul Hameed Hamid Mohammad Agha said.

In Kabul, hit by a series of deadly attacks during the election campaign, hundreds of people lined up in the open air to vote despite the insurgents' promise of violence.

"I'm not afraid of Taliban threats, we will die one day anyway. I want my vote to be a slap in the face of the Taliban," housewife Laila Neyazi, 48, told AFP.

Poll security was a major concern following the attacks in Kabul, most recently a suicide bombing on Wednesday that killed six police officers.

Interior Minister Omar Daudzai said all 400,000 of Afghanistan's police, army and intelligence services were being deployed to ensure security around the country.

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, leaving local forces to battle the resilient Taliban insurgency without their help.

In the western city of Herat, a queue of several hundred people waited to vote at one polling station, while in Jalalabad in the east, voters stood patiently outside a mosque.

Voters also lined up in Kandahar city, the southern heartland of the Taliban, with some women among the crowd in contrast to the 2009 election when turnout was very low due to poor security.

The country's third presidential election brings an end to 13 years of rule by Karzai, who has held power since the Taliban were ousted in 2001.

Karzai, who is constitutionally barred from standing again, voted for his successor in a school near the presidential palace in Kabul.

"I urge the Afghan nation to go to the polling stations despite the rain, cold weather and enemy threats... and to take the country another step towards success," he said.

- No clear favourite -

Around 13.5 million people are eligible to vote from an estimated total population of 28 million, with polls due to close at 4:00 pm.

As well as the first round of the presidential election, voters will also cast ballots for provincial councils.

The front-runners to succeed Karzai are former foreign minister Zalmai Rassoul, Abdullah Abdullah — runner up in the 2009 election — and former World Bank academic Ashraf Ghani.

All three voted soon after polls opened.

There is no clear favourite and if no candidate wins more than 50 percent of the vote in the first round — preliminary results for which will be announced on April 24 — a run-off is scheduled for late May.

Massive fraud and widespread violence marred Karzai's re-election in 2009 and a disputed result this time would add to the challenges facing the new president.

Whoever emerges victorious must lead the fight against the Taliban without the help of more NATO troops, and also strengthen an economy reliant on declining aid money.

The election may offer a chance for Afghanistan to improve relations with the United States, its principal donor, after the mercurial Karzai years.

Relations fell to a new low late last year when Karzai refused to sign a security agreement that would allow the US to keep around 10,000 troops in Afghanistan to train local forces and hunt Al-Qaeda.

The eve of the poll was overshadowed by the killing of award-winning Associated Press photographer Anja Niedringhaus, shot by a police commander in eastern Afghanistan.

Niedringhaus, 48, was the third journalist working for international media to be killed in Afghanistan during the election campaign, after Swedish journalist Nils Horner and Sardar Ahmad of Agence France-Presse.