Nine people were killed and about two dozen others wounded in an attack targeting the Kabul office of Ghani's running mate, Amrullah Saleh.

The violence came on the first day of campaigning for the upcoming presidential elections, serving as a grim reminder of Afghanistan's woeful security situation and the sort of mayhem and murder that have beset previous polls.

The attack began around 4:40 pm (1210 GMT), when a huge blast struck near the Green Trend office, a youth- and reform-focused civil society organisation Saleh heads. He escaped without serious injury, his office said.

The attack lasted about six hours and ended with all attackers killed, according to the interior ministry. No group immediately claimed responsibility.

Ghani's peace minister, Abdul Salam Rahimi, gave the toll of nine -- up from two dead initially -- and said 25 people were wounded.

Earlier on Sunday, a buoyant Ghani kicked off his campaign by insisting "peace is coming," after nearly 18 years of conflict, and that pivotal talks with the Islamist extremist Taliban would take place.

He is hoping to fend off challenges from 17 other candidates to score a second term at twice-postponed presidential elections now slated for September 28.

On Saturday Rahimi, said direct talks would take place with the Taliban within two weeks as part of a larger, US-led push for peace.

Such a development could be crucial, as the Taliban -- who now control or influence about half of Afghanistan -- have so far refused to speak to Ghani's government. They consider the Kabul administration illegitimate.

- Despondent voters -

War aside, the country faces a host of major issues ahead of the election, including rocketing crime, a lacklustre economy, soaring unemployment, and crumbling infrastructure.

Voters are despondent about the prospects of a fair election. Many worry about a repeat of violent attacks on previous polling stations by the Taliban and other insurgent groups trying to undermine Afghanistan's fragile democracy.

Ghani insisted this year's vote would be "clean", but distrust is rife.

Sayed Jan, a 27-year-old student, said he won't be voting as he has lost faith since the 2014 election that was mired in allegations of fraud and ballot stuffing.

"We have been betrayed by the candidates in the past. We cannot trust them this time," he told AFP.

"We need peace in Afghanistan instead of elections. Even if I vote, the election will be fraudulent."

- 'Opportunities for peace' -

In Kabul, security forces fanned out across the city as leading candidates held rallies.

Ghani's top rival is Abdullah Abdullah, who currently serves as the president's chief executive under an awkward power-sharing arrangement brokered by the US after the 2014 election.

"It is our national and religious duty to take advantage of any opportunities for peace," Abdullah told a campaign rally.

One crucial issue is that the elections happen at all: they were postponed twice this year and further delays could lead to more distrust.

Despite Ghani's claim that a summit between his government and the Taliban would take place shortly, the insurgents said they would only talk to Kabul after the US had announced a timeline for a withdrawal of foreign forces -- a major part of any deal.

"The Kabul administration will be considered a political side, just like others, and not a government," Taliban political spokesman Suhail Shaheen wrote in Pashto on Twitter.

Diplomatic sources have told AFP the Afghan-Taliban talks are scheduled to begin in Oslo on August 7.

US peace envoy Zalmay Khalilzad said Saturday that new "intra-Afghan" negotiations would only take place after the US and Taliban had concluded their own agreements.

Washington is hoping for a political agreement with the insurgents ahead of the September presidential election.

Other presidential candidates include Ghani's former national security advisor Hanif Atmar and former warlord Gulbuddin Hekmatyar, nicknamed the "Butcher of Kabul" for his alleged role in the killing of thousands of people in the capital in the 1990s.