The Taliban have vowed to disrupt the polls on August 20.
James Bays, Al Jazeera's correspondent in Kabul, said there were fears that suicide bombers were already in place in Kabul, the capital, to carry out attacks on election day.
"Intelligence sources are telling Al Jazeera they have some information that maybe there are more suicide bombers hidden in the city," Bays said.
"They're trying to stop anyone else coming in, they are searching a lot of the cars [coming into Kabul], but they fear that some may already be here."
The series of attacks launched on Tuesday included 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 the attack, including a Nato soldier and two UN workers.
Security officials said that at least 52 people were wounded in the blast, the majority of them civilians.
Aleem Siddique, a spokesman for the UN mission in Kabul, said that a UN vehicle had been caught up in the strike, but had not been the main target.
The road is often attacked by the Taliban due to the large number of compounds and military bases alongside it.
The Taliban claimed responsibility for the raid, which came hours after two mortars hit the presidential palace in Kabul and nearby police headquarters.
No casualties were reported.
Election workers killed
Marwan Bishara, Al Jazeera's senior political analyst, said that the elections would be a key test in the fight between the US and the Taliban.
"The idea of elections, within a comprehensive American strategy, is meant to underline that Afghanistan has minimum stability, has minimum security and has minimum commonality of thought that unites the country," he said.
"But the Taliban is trying to do exactly the opposite. It is trying to show that there is no stability in the country - that it is divided and lacks security."
In the country's usually peaceful north, attacks killed five people, including an election candidate and three electoral workers.
A roadside bomb ripped through a vehicle in the northern province of Badakhshan, killing three electoral workers, including the district's election chief, and their driver.
In a separate attack in the northern Jawzjan province, Abdul Rahim, a provincial candidate in the elections, was killed in a Taliban ambush.
Six rocket attacks were also reported in Jalalabad in the east, injuring at least nine people, including four children, and a suicide bomber attacked a military post in the southern Uruzgan province, killing five - three Afghan soldiers and two civilians.
In Paktia province, a school being used as a polling station was burned down by the Taliban, but no one appeared to have been hurt.
About 17 million people are eligible to vote in Afghanistan's elections. They will be voting for a president and 420 councillors across 34 provinces.
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,
"Our efforts alongside our Afghan security partners will focus on protecting the people of Afghanistan from the insurgents so that the population can freely exercise their right to choose their next president and their provincial representatives," Brigadier-General Eric Tremblay, a spokesman for Nato, said.
But the Taliban have demanded that Afghans boycott the polls, threatening to cut off the fingers of those who vote and saying they will attack polling stations.
Opinion polls are showing a lead for Hamid Karzai, the incumbent president, with 45 per cent of the ballot, but not an outright majority.
Without an outright majority, Karzai would be forced into a run-off with his closest challenger, predicted to be Abdullah Abdullah, his former foreign minister.