Seven million Afghans have voted in a presidential election run-off.
The election commission chief Ahmad Yousuf Nuristani said this was roughly the same as in the first round of voting in April.
The election pitted former anti-Taliban fighter Abdullah Abdullah against ex-World Bank economist Ashraf Ghani after neither secured the 50% majority needed to win outright in the first round on 5 April.
It is viewed as a decisive test of Afghanistan's ambitions to transfer power democratically for the first time in its tumultuous history.
The vote pits former anti-Taliban fighter Abdullah Abdullah against ex-World Bank economist Ashraf Ghani after neither secured the 50% majority needed to win outright in the first round on 5 April.
Voters were not put off by a couple of rockets landing in the capital and some other explosions.
Long queues were formed at polling station before voting began at 7am.
As most foreign troops leave by the end of 2014, whoever takes over from Mr Karzai will inherit a troubled country.
There is an increasingly violent Taliban insurgency and an economy crippled by corruption and the weak rule of law.
The process has been fraught with accusations of fraud by both candidates.
Many fear a close outcome will make it less likely the loser will accept defeat.
It could possibly drag Afghanistan into a risky, protracted stand-off over the vote.
The Taliban may prove a formidable obstacle.
The insurgents, now at the height of their summer offensive, have warned people not to vote in an election they have condemned as a US-sponsored charade.
One person was reported to have been wounded in the blasts in Kabul.
On 6 June, Mr Abdullah survived an assassination attempt when two bombs exploded outside a Kabul hotel where he had just held a rally. Twelve people were killed.
The high turnout of nearly 60% in the first round of the election was a major defeat for the Taliban.
Observers expect fewer than five million voters this time, partly due to security concerns.
Officials in Kabul are haunted by the prospect of a close outcome that could furnish the losing candidate and his supporters with an excuse to reject defeat, and, in the worst scenario, propel the country back into war along ethnic lines.
Both candidates set the stage for complaints repeatedly accusing electoral organisers of incompetence and bias.
The United Nations has appealed to candidates to refrain from attacking the organisers to safeguard the process.
Mr Abdullah polled 14 percentage points ahead of Ghani in the first round with 45% of the vote, but Ghani, who is ethnic Pashtun, stands to gain a portion of the Pashtun vote that was splintered in the first round.
Pashtuns are Afghanistan's biggest ethnic group, making up about 45% of the population. While Mr Abdullah is partly Pashtun, he is identified more with the ethnic Tajik minority.
The chances of an equal split between candidates are hard to gauge because there are few reliable polls.
ACSOR research centre, asking respondents to choose between Abdullah and Ghani, predicted a 50:50 split shortly before the first round.