KABUL, Afghanistan — In the first official report of partial results from the Afghan presidential election, the candidates Abdullah Abdullah and Ashraf Ghani appeared to be leading a race in which a runoff election was increasingly certain, according to data released by the Independent Election Commission on Sunday.

The commission warned that these early results, accounting for 10 percent of the votes cast in 26 of Afghanistan’s 34 provinces, could well change as tabulating continued over the coming weeks. The votes will be fully counted by April 24, and a runoff election would be held no sooner than May 28, officials have said.

The results could well be affected, possibly significantly, by widespread fraud at the polls. The election complaints commission said on Sunday that it had received so many serious fraud complaints that it might have to extend the time needed to adjudicate them. The commission said 870 instances of fraud had been classified as serious enough to affect the outcome of the April 5 election, more than the 815 recorded in 2009.

The early results prompted an outcry from the six candidates who did not rank among the top two. After meeting privately to discuss the figures, they issued a joint statement rejecting the partial results as premature, and described the election commission’s decision to release them as an “unforgivable crime.”