Further highlighting the security fears on Wednesday, at least three people were killed in a gun battle after a number of armed men raided a bank in the Afghan capital.
The Afghan authorities initially said the incident was an attempted robbery, but the Taliban quickly claimed responsibility for the attack.
Police said that the dead were members of the group that carried out the raid, adding that three police officers were injured.
Al Jazeera's James Bays, reporting from Kabul, said the siege went on for more than four hours before police announced the fighting was over.
"We're told that the gunmen had planted rigged explosives around the building and that's where the policemen were wounded," he said.
"The Taliban says it is their operation. A Taliban spokesman told Al Jazeera there [were] five attackers equipped with suicide vests."
Bays said officials tried to keep reporters away.
"They are trying to prevent journalists here from filming in a way that I've never seen before in Afghanistan."
The Afghan government has called for an international media blackout on reporting violence on the day of the election, amid fears that security concerns could keep voters at home.
The government's appeal to the media was contained in a foreign ministry statement.
|Afghans returning from Pakistan to cast their ballot on the eve of the vote [EPA]
"All domestic and international media agencies are requested to refrain from broadcasting any incident of violence during the election process," it said.
Also on Wednesday, the US military announced the deaths of six soldiers, pushing to at least 32 the number of US troops to die in August so far.
Wednesday's violence followed a series of attacks on Tuesday, including a suicide car bombing east of the capital along the Jalalabad-Kabul road, which targeted members of Nato's International Security Assistance Force (Isaf).
Eight people were killed in that attack claimed by the Taliban, including a Nato soldier and two UN workers, and 52 were wounded. The Taliban claimed responsibility.
The Nato-led military force in Afghanistan has vowed to make every effort to protect voters, saying that more than 100,000 international troops will refrain from offensive operations on election day.
But the Taliban has demanded that Afghans boycott the polls, threatening to cut off the fingers of those who vote and saying they will attack polling stations.
A statement from the Taliban on Wednesday said Taliban fighters were closing roads across the country, the Reuters news agency reported.
|The Taliban has vowed to disrupt the country's presidential election [AFP]
"From today onwards until the end of tomorrow, all main and secondary roads will be blocked for traffic and the mujahidin will bear no responsibility for whoever gets hurt," the statement said.
Maiwand Karbuk, a 29-year-old resident of Kabul, told Al Jazeera on Wednesday that he will not vote.
"[The Taliban] have given a lot of warnings. No one will take that risk to go to polling stations," he said.
He also said the elections would not help Afghanistan.
"I don't think these elections are transparent. In my opinion, the person who is in favour of Western countries will ultimately win, whether there are elections or not."
Opinion polls are showing a lead for Hamid Karzai, the incumbent president, with 45 per cent of the ballot.
But without an outright majority of 50 per cent, Karzai would be forced into a run-off with his closest challenger, which is expected to be Abdullah Abdullah, his former foreign minister.