The Taliban, hardline Islamists bent on toppling the government, have deployed fighters countrywide to disrupt an election they brand a U.S.-backed sham. Dozens of people have been killed in a spasm of violence leading up to the vote.
A veteran Associated Press photographer was killed and a senior correspondent of the same news agency was wounded on Friday when a policeman opened fire on the two women in eastern Afghanistan as they reported on preparations for the poll.
More than 350,000 Afghan troops have been put on duty to thwart attacks on polling stations and voters. The capital, Kabul, has been sealed off from the rest of the country by rings of roadblocks and checkpoints.
Kandahar, cradle of the Taliban insurgency, was in virtual lockdown ahead of the vote. Residents were advised to stay home.
Hamid Karzai, the incumbent, is not allowed to run for the presidency again by the constitution, but is widely expected to retain his hold on politics through politicians loyal to him.
Voters will inevitably be looking back at Afghanistan's progress since 2001 when U.S.-led forces toppled the Taliban who were harbouring Osama bin Laden and al Qaeda.
Those 13 years have seen seemingly endless bloodshed - at least 16,000 civilians, 3,500 foreign troops and thousands of Afghan soldiers have been killed.
Billions of dollars have been spent rebuilding the country.
"The Karzai government has to be given credit for some limited achievements on human rights in very difficult conditions," Amnesty International said in a statement.
"But the situation for millions of Afghans remains dire, and even the progress we have seen is very fragile."
With no clear frontrunner, the process is likely to drag on for weeks, if not months, particularly if there is a run-off.
Any delay would leave little time to complete a crucial pact between Kabul and Washington to keep up to 10,000 U.S. troops in Afghanistan beyond 2014 - after most foreign troops pull out.
"The whole future of Afghanistan is at stake," Franz-Michael Mellbin, the EU's special representative in Kabul, told Reuters. "It's crucial ... the Afghans come out and vote in large numbers and give political legitimacy, and the aftermath of the elections will be crucial because we need a stable government."
"WE SHOULD GO OUT AND VOTE"
Of the eight candidates, the three favourites are former foreign ministers Abdullah Abdullah and Zalmay Rassoul, and former finance minister Ashraf Ghani.
If, as predicted, no one wins more than 50 percent on Saturday, there will be a second round between the two leading contenders. That run-off would be held on May 28, spinning out the process into the holy month of Ramadan when offices are shut for much of the day and life slows to a crawl.
Preliminary results from the first round are due nearly six weeks after polling day, partly because of Afghanistan's mountainous terrain which will require about 3,000 donkeys to carry ballot boxes from the most inaccessible areas.
And yet most people expect the election will be better run than the chaotic 2009 vote, when 20 percent of the ballots were thrown out as fraudulent. Many Afghans see the election as an important turning point in their lives.
"We always complain about our president and his government's shortcomings but we never criticise ourselves," said Yaar Mohammad, a real-estate agent in the eastern city of Jalalabad.
"If we want a new leader to work for peace and improve our lives," he said, "then we should go out and vote and not be scared of the Taliban."
But some were worried about security in a country which recorded the highest number of conflict-related civilian deaths last year since 2001.
"We have war in Kandahar. Taliban, weapons - these could be used in the process," said Hayatullah, who trains election staff in Afghanistan's second city. "We are worried about that."
"Every day we feel fear," he said, recalling an attack on the election commission headquarters in Kabul last week. "We are afraid that we may be attacked too."