People in Afghanistan are voting for a new president in what will be the nation's first ever transfer of power through the ballot box.
Turnout is reported to be brisk despite heavy rain and worries over security.
A massive operation is under way to thwart the Taliban which has vowed to disrupt the election.
Eight candidates are vying to succeed Hamid Karzai, who is barred by the constitution from seeking a third consecutive term as president.
Heavy rain was falling in Kabul early on Saturday and BBC correspondents said young voters in particular were defying the conditions and the security threats.
Traffic was prevented from entering the Afghan capital from midday on Friday, with police checkpoints erected at every junction.
In parts of the capital voters could be seen queuing an hour before polls opened. The BBC's correspondent in northern Afghanistan says that large queues had also formed there as people wait to cast their ballots.
However, some polling stations in the provinces of Herat in the west and Kapisa, north-east of Kabul, were closed because of a combination of the bad weather and security risks.
There were also reports elsewhere of several polling centres not receiving ballot materials in time and of ballot papers running out at some stations.
However the BBC reports that international observers are increasingly optimistic that both the tight security and a number of new guarantees against fraud will make this a fairer election than Afghanistan has seen before.
There are eight candidates for president, but three are considered frontrunners - former foreign ministers Abdullah Abdullah and Zalmai Rassoul, and former finance minister Ashraf Ghani Ahmadzai.
However, no candidate is expected to secure more than the 50% of the vote needed to be the outright winner, which means there is likely to be a second round run-off on 28 May.
In the latest in a string of deadly attacks that marred the lead-up to the election, award-winning German photographer Anja Niedringhaus was killed and veteran Canadian reporter Kathy Gannon was injured when a police commander opened fire on their car in the eastern town of Khost on Friday.