There is a huge security operation in Afghanistan as presidential candidates prepare for Saturday's vote.
The new president will succeed Hamid Karzai, who has been in power since the 2001 fall of the Taliban but is constitutionally barred from seeking a third consecutive term.
It should be the first time that power is democratically transferred.
But pitfalls lie ahead, especially the threat of Taliban violence as Nato prepares to withdraw later this year.
Security has been tightened across the country with nearly 200,000 troops deployed to prevent attacks by the Taliban, who have threatened to disrupt the poll.
The BBC's Afghanistan correspondent David Loyn says the election is being protected by the biggest military operation since the fall of the Taliban.
Reporting restrictions are in place, limiting what can be broadcast about the candidates.
If nobody wins more than 50 percent of the vote in this round, a run-off election will be necessary.
The BBC's Lyse Doucet in Kabul says that the third presidential race since the fall of the Taliban is certain to be marred by rigging, recrimination and violence.
Our correspondent says that the run-up to this historic poll has already been the bloodiest, and fears of electoral fraud are pronounced.
But a new political culture is slowly emerging, our correspondent says.
There are eight candidates for president, including former foreign ministers Abdullah Abdullah and Zalmai Rassoul, and former finance minister Ashraf Ghani Ahmadzai.
Correspondents say the election may give the US a new chance to repair relations with Kabul, which are moribund after more than 12 years of war and repeated rows between the White House and President Karzai.
A stable and acceptable political transition is "critical to sustaining international support for Afghanistan", White House press secretary Jay Carney said on Monday.
Relations between the president and Washington plunged to new lows late last year when the Afghan leader refused to sign a bilateral security agreement that would allow up to 10,000 troops to stay in his country after the Nato combat mission ends.
The troops staying on would train, advise and assist Afghan security forces and conduct counter-terrorism missions.
Correspondents say that President Karzai is eager to ensure that his legacy does not include a commitment to allow the continued deployment of international troops in his country.
It is unclear what role he sees for himself after the election.
Much may depend on whether a clear front-runner emerges at the head of the eight presidential hopefuls.
However, Saad Mohseni, who heads the Moby Group that includes the popular Tolo TV, said its polls indicated the number of undecided voters may be as high as 30-40 percent.
With a clear winner in the first round unlikely, a result may not be known until the summer.