By Mirwais Harooni and Jessica Donati

KABUL (Reuters) - Initial election results put Afghan opposition figure Abdullah Abdullah in the lead on Sunday, but with less than 10 percent of votes counted and widespread allegations of fraud, there was no clear indication of who would succeed Hamid Karzai.

Results based on 10 percent of votes from 26 out of 34 provinces showed Abdullah with 41.9 percent and Western-leaning academic Ashraf Ghani second with 37.6 percent. A third candidate, Zalmay Rassoul, backed by two of Karzai's brothers, trailed far behind with 9.8 percent.

"I want to make clear that the results could change in future, as we announce the results with additional percentages of the vote, and this is not the final result," said Independent Election Commission (IEC) chairman Ahmad Yousuf Nuristani.

Afghanistan's allies praised the April 5 vote as a success because of a high turnout estimated at 60 percent of 12 million eligible voters and the failure of the Taliban to stage high-profile attacks.

But widespread fraud could undermine the legitimacy of an election meant to usher in Afghanistan's first democratic transfer of power, as Karzai steps down after more than 12 years in power and as Western forces prepare to leave.

If no candidate secures more than 50 percent of valid ballots, the top two will go into a run-off.

Abdullah has said he had already discussed joining forces with other candidates for a run-off, including Rassoul.

"Our program will be to form an inclusive government ... We should use any capacity that exists in this country," Abdullah told Reuters on Sunday.

"So we are in contact not with just one candidate, but also other candidates and politicians in the country."

Ghani said it was far too early to talk about a victory for his rival as the partial results were not significant.

"We are in a 100-minute game and we have only done 10 minutes ... the result will change," he said.

COMPLAINTS RISE

The Independent Election Complaints Commission (IECC) has recorded 870 "Priority A" complaints - considered serious enough to affect the outcome of the election. That is more than the 815 recorded in 2009, although the increase could reflect a greater willingness or ability of observers and voters to complain.

Story continues

Overall, the IECC received 3,724 complaints, up from 3,072 in 2009. That may rise as more complaints reported in the provinces reach Kabul.

"There is a possibility, in order to review the high number of complaints accurately, that we may expand the timeframe for reviewing complaints in provinces for some days," said IECC spokesman Nader Mohseni.

Final results are due on May 14.

Video clips of people stuffing ballot boxes are circulating on the Internet, but it unclear which candidate would have benefited most from such activity.

Urban participation in the election was unexpectedly high, but it is unclear to what extent rural voters were deterred by the Taliban and what role state officials, including police, had in encouraging people to back a particular candidate.

The United Nations, which administers the fund that is paying for the election commission and the complaints body, said it was still too early to call the election.

"Until the final results are announced by the IEC, stakeholders should be careful in drawing premature conclusions so as not to create inaccurate expectations," said U.N. special envoy Jan Kubis.

(Additional reporting by Hamid Shalizi; Editing by Robin Pomeroy)