Voting began on Saturday in Afghanistan's presidential election, which will mark the first democratic transfer of power since the country was tipped into chaos by the fall of the hardline Islamist Taliban regime in 2001.
Taliban insurgents had launched a spate of attacks that killed dozens in the run-up to the vote, which they brand as a US-backed sham.
Early on election day, four voters were wounded in an explosion at a polling station in the southeastern province of Logar, just a few hours after polls opened.
"The blast took place close to a polling station which is a school building and wounded four voters, one critically," Abdul Hameed, governor of the province's Mohammad Agha district, told Reuters.
About 12 million are eligible to vote in the election.
There are eight candidates contesting, with former foreign ministers Abdullah Abdullah and Zalmay Rassoul, and former finance minister Ashraf Ghani the favourites.
Hamid Karzai, the incumbent, is not allowed to run for the presidency again by the constitution.
But, after 12 years in power, Mr Karzai is widely expected to retain influence through politicians loyal to him.
Mr Karzai urged Afghan citizens to participate in elections that will take the country towards stability as he cast his vote Saturday morning.
Voting began at 7:30 am and polls close at 4:30 pm local time.
Preliminary results will be announced on April 24, according to the Independent Election Commission of Afghanistan, with the final tally scheduled for May 14.
If no candidate wins more than 50 per cent of votes -- a scenario the head of U.S. forces in the country views as probable -- a run-off between the top two candidates would take place around the end of May.
The Taliban is boycotting the polls and vowed to use "all force" to disrupt the process that will be monitored by more than 260,000 people and will cost Asia's poorest economy and its allies US$136 million.
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.
Most people expect the election will be better run than the chaotic 2009 vote that handed Mr Karzai a second term amid massive fraud and ballot stuffing.
But it could take months - perhaps until October - for a winner to be declared at a time when the country desperately needs a leader to stem rising violence as foreign troops prepare to leave.
Any delay would leave little time to complete a pact between Kabul and Washington to keep up to 10,000 U.S. troops in the country beyond 2014, after the bulk of the American force, which currently stands at around 23,500, has pulled out.
Mr Karzai has rejected the agreement, but the three frontrunners to succeed him have pledged to sign it.
Without the pact, far weaker Afghan forces would be left on their own to fight the Taliban, who have mounted an increasingly bold and violent campaign against the Kabul government.
Uncertainty over the outcome could also stall crucial foreign aid and economic reform, foment ethnic tensions and leave a political vacuum in which the Taliban could gain ground.
