Afghanistan's presidential candidates held major rallies in Kabul on Sunday marking the start of an election campaign to appoint Hamid Karzai's successor, as the killing of a frontrunner's aides highlighted the security threat surrounding the poll. Gunmen shot dead two aides of Abdullah Abdullah, a former foreign minister, in the western city of Herat on Saturday, officials said. The attack comes as the country prepares for its first democratic transfer of power, with the April 5 election viewed as a key test of the effectiveness of the 350,000-strong Afghan security force as foreign troops prepare to exit the country. A dispute between Kabul and Washington over whether a small force of US soldiers stays behind beyond 2014 is likely to dominate the campaign. In the capital on Sunday, thousands of people, mostly men, gathered in giant wedding halls where candidates delivered speeches and called on war-weary Afghans to vote for them. Ashraf Ghani, a 64-year-old academic and internationally known intellectual, told one packed hall: "Reforms will begin with us: myself, Mr Dostum and Mr Danish." He was referring to his running mates, the former Uzbek warlord General Abdul Rashid Dostum and ethnic Hazara tribal chieftain Sarwar Danish. Security was tight at the rallies, which were guarded by the Afghan national army. But despite the army's presence, the killing of Abdullah's aides weighed heavily on some people's minds. Voters 'concerned' Arefa Alizada, an 18-year-old Abdullah supporter who attended one of the rallies, said: "I am concerned about security of the election, especially after I heard that two campaigners were killed yesterday. If it worsens, me and many other people won't be able to vote." Afghanistan has been gripped by a deadly insurgency for the past 12 years. Most US and NATO troops are set to leave at the end of this year, leaving Afghans in charge of their own security. A dispute between Kabul and Washington over whether a small force of US soldiers stays behind beyond 2014 is likely to dominate the campaign. Karzai was expected to sign a bilateral security agreement (BSA) late last year, which would allow about 10,000 US troops to be deployed in the country after NATO withdraws by December. But he has stalled and said his successor might now complete negotiations -- plunging relations with the US, Afghanistan's key donor, to a fresh low. Karzai has ruled the country since the fall of the Taliban in 2001, surviving assassination attempts and the treacherous currents of Afghan political life as billions of dollars of military and development aid poured into the country. He is barred from seeking a third term, leaving an open field to compete in the April 5 vote, which is likely to trigger a second-round run-off in late May between the two strongest candidates. Abdullah, the suave opposition leader who came second to Karzai in the chaotic and fraud-riddled 2009 election, is tipped to go through to the next round. Former finance minister Ghani, Karzai loyalist Zalmai Rassoul and the president's low-profile elder brother Qayum Karzai are also considered heavyweights. In comments likely to cause further friction with his NATO allies, Karzai criticised their conduct during the 12-year conflict in an interview with Britain's Sunday Times in which he described the Taliban as "brothers" and the US as "rivals". Karzai told the newspaper that "the US-led Nato mission in terms of bringing security has not been successful, particularly in Helmand", a southern stronghold of Taliban militants. "We have immense respect for the life of Nato soldiers lost in Afghanistan and strong disagreement for the way US conducted itself in Afghanistan," he said. Western and Afghan officials say all 11 candidates support the BSA but, except for Abdullah, they have declined to say so publicly for fear of clashing with Karzai. Taliban insurgents have threatened to target the campaign, and the Afghan police and army face a major challenge with little support from the dwindling number of NATO troops. Disputes over millions of fraudulent ballots led to a major crisis after voting in 2009, before Abdullah pulled out of the run-off, leaving Karzai to take power. Election organisers are again expected to be busy with complaints of fake votes, ballot-box stuffing and polling booths unable to open due to voter intimidation. "Holding elections is not an easy job in the current situation in Afghanistan," Yousuf Nuristani, chairman of the Independent Election Commission, told candidates recently. "We hope you carry out your election campaigns in accordance with the law and in a good environment."