By Maria Golovnina and Mirwais Harooni
KABUL, April 7 (Reuters) - Preliminary tallies from Afghanistan's presidential election showed former foreign minister Abdullah Abdullah leading in parts of Kabul on Monday, but with ballot counting likely to last weeks it was far too early to predict a winner.
The two other frontrunners alleged serious fraud in the April 5 vote, which all being well will lead to Afghanistan's first democratic transfer of power as incumbent Hamid Karzai prepares to step down after more than 12 years in the office.
Trucks stacked with plastic ballot boxes began to trickle into the capital on Monday, as officials prepared to open a centre to tally votes from across the rugged country.
Given the United State's plan to withdraw most of its troops by the end of the year, the longer Afghanistan has to wait before a new leader is installed the greater is the risk of instability either from the Taliban insurgency, or rivalries between factions in a country riven by ethnic and tribal fault lines.
"We are trying to start the process as soon as possible," said Noor Mohammad Noor, a spokesman for the Independent Election Commission. "It's a long process. It will take time."
If none of the candidates score over 50 percent it will take even longer before a winner is declared, as there will be a second round run off at the end of May at the earliest.
Preliminary results are not due until April 24, but a tour of Kabul polling stations showed that Abdullah was firmly in the lead, confirming the suave former anti-Soviet resistance fighter's popularity in the capital.
Running second came Ashraf Ghani, a former World Bank official with a programme of radical economic reform. Former finance minister Ghani should score well in the north, as his running mate is Abdul Rashid Dostum, a feared former warlord who holds sway over much of the region.
Trailing in third was another former finance minister, Zalmai Rassoul. He is believed to have Karzai's backing, and as a consequence is expected to do better in the Pashtun-dominated south, where the Karzai family has strengthened its influence over the years.
As part of Afghanistan's efforts to make the election more transparent, polling stations at schools and offices across the country post results on the door for the public to see.
At the Sayed Jamaluddin High school polling station in western Kabul, for example, Abdullah had received 190 ballots, Ghani 23 and Rassoul 11.
The picture was similar at about a dozen other schools visited by Reuters in the west and north of Kabul.
Kabul accounts for at least 20 percent of Afghanistan's electoral base, so a strong result in the capital could be key to deciding the elections' outcome. There are 28,500 polling centres across Afghanistan.
Rekindling memories of the mass fraud that marred the previous presidential election in 2009, both Ghani and Rassoul said they had received reports of violations and submitted them to the Election Complaints Commission (ECC).
"There are reports of serious fraud in several locations but all is documented and will be passed on to ECC for investigation," Ghani posted on his Twitter account.
An aide for Rassoul said: "We have filed complaints." The extent of the suspected fraud was not immediately clear and the ECC was expected to take weeks to check the allegations.
International and Afghan leaders have praised the Saturday vote as a success because of a higher than expected turnout - estimated at about 60 percent - as well as the failure of the Taliban insurgents to disrupt it significantly.
Although the Taliban failed to pull off major attacks on election day itself, some people fear the insurgents are preparing to disrupt the ballot-counting process. On Sunday, a roadside bomb killed two election workers and a policeman and destroyed dozens of ballot papers in northern Afghanistan. (Additional reporting by Hamid Shalizi and Jessica Donati, Writing by Maria Golovnina; Editing by Simon Cameron-Moore)