KABUL, Afghanistan -- A suicide bombing killed six policemen at the Afghan Interior Ministry compound in one of the capital's most heavily fortified areas Wednesday, part of a recent escalation in violence in the heart of Kabul.

The bloodshed is threatening to scare voters away from the polls as Afghans worry security forces unable to guard areas previously considered safe won't be able to protect them on election day. The Taliban have launched a campaign of violence to disrupt Saturday's vote for a new president and provincial councils.

Many voters have defiantly said they would go to the polls despite the violence, but Wednesday's attack was a last straw for some.

Mohammad Ramin, an 18-year-old who has a photo store near the site of the blast, said he registered to vote for the first time last week but was too scared to go to the polls now.

"I am so disappointed, but I am not going to vote on election day because of the bad security," he said. "I don't want anybody in my family to go either."

The bomber, wearing a military uniform, passed through several checkpoints to reach the ministry gate before detonating his explosives in the midst of other uniformed men entering the compound, according to the Interior Ministry, which oversees the Afghan police.

Within minutes of the blast, Taliban spokesman Zabihullah Mujahid claimed responsibility for the attack. It came soon after he issued an emailed statement to the media warning of more violence ahead of the elections.

The Interior Ministry has the lead in protecting the polling stations on Saturday, when voters will choose a new president in the first democratic transfer of power since Hamid Karzai was selected for the job after the 2001 U.S.-led ouster of the Taliban. Karzai is constitutionally barred from seeking a third term.

Many Afghans have expressed defiance, saying they will exercise their right to vote despite a series of election-related attacks.

The militants have also increasingly been targeting Westerners. In recent weeks, the Taliban also have claimed responsibility for attacks against a luxury hotel, a foreign guest house, a Swedish journalist and a Lebanese restaurant popular with foreigners.

Mohammad Karim, who was walking toward the gate to leave the Interior Ministry compound on Wednesday, said he was blown back by the force of the blast. Police then rushed him and others into a safe room.

Police officer Baryalai, who like many Afghans uses only one name, said the bombing took place near a bank close to the entrance gate. Police officers collect their paychecks at the bank.

Farida Hashimi, a female police commander, wept for the policemen killed in the explosion, including one named Tamim.

"One of the police who died was my friend. They were all my colleagues, but Tamim was my friend, my brother," she said at the site of the explosion. She criticized the lax security that allowed the bomber to walk past three different check posts.

Hashimi said the Interior Ministry explosion did not bode well for Saturday's elections. Afghans will worry about the ability of government forces to protect polling stations, she said, when they are unable to protect their own headquarters.

"It will have a very bad effect on the elections," Hashimi said. "Or course people will wonder how they can keep polling places safe if a bomber can enter this very important government compound."

Mujahid, the Taliban spokesman, warned Afghans to stay away from polling stations, saying election workers and polling centres would be targeted.

Security and electoral officials have vowed not to let the Taliban derail the elections while conceding it is impossible to prevent the Islamic militants from waging acts of violence.

Earlier Wednesday, an Afghan official said Taliban gunmen killed nine people, including a candidate running for a seat in a provincial council, who had been abducted in northern Afghanistan.

Sar-i-Pul Provincial Governor Abdul Jabar Haqbeen said authorities received word that the candidate, Hussain Nazari, and the others were killed overnight by their abductors. They were seized by the Taliban three days earlier while travelling to the provincial capital.

Haqbeen said they recovered four bodies and one man who was wounded, while the five other bodies, including the candidate's, were in a remote area and had yet to be found.

Meanwhile, Abdullah Abdullah, who was Karzai's main rival in his disputed re-election in 2009 and is a leading contender in the current presidential race, said he was optimistic he could win in the first round on Saturday.

"We do not dismiss other scenarios, but it is most likely that there will be one-round elections and there will not be a long period of uncertainty," Abdullah said at a news conference to mark the last day of official campaigning.

Abdullah and two other presidential candidates -- Zalmai Rassoul and Ashraf Ghani Ahmadzai -- are considered to have the best shot at winning out of a field of eight contenders. Many experts have predicted a runoff between the top two vote-getters.

At a rally in Kabul, Rassoul urged all Afghans to exercise their right to vote.

"For me, moving forward the democracy process is more important than winning the elections, so regardless of who you are going to choose, I call up on you to vote on election day and make this process successful," he said.