Both candidates promise a fresh start with the West, vowing to sign a security pact with the United States that has been rejected by President Hamid Karzai, but their fierce rivalry has raised the possibility of divisive campaigning in what so far has been a relatively peaceful vote.
With 10 percent of the ballots counted, Abdullah Abdullah, who was Karzai's main rival in his fraud-marred re-election in 2009, had 41.9 percent of the vote. Ashraf Ghani Ahmadzai, a former finance minister and World Bank official, followed with 37.6 percent. Zalmai Rassoul, another former foreign minister widely considered as Karzai's pick, was a distant third with 9.8 percent.
Karzai himself was constitutionally barred from seeking a third term.
Officials cautioned the vote count could change as full preliminary results won't be due until April 24, but the early numbers suggest none of the eight candidates likely will get the outright majority needed to avoid a runoff.
Shortly after officials announced the results, Abdullah told The Associated Press he has held talks with Rassoul but it was premature to discuss a possible alliance. The 53 year old said he will seek a unity government if elected, but he only saw one role for Ghani.
"Dr. Ghani could serve as a loyal opposition. That's also a service to the nation," Abdullah said in an interview at his home in Kabul.
Ghani, however, remained confident that he would be in first place at the final tally. He dismissed talk of a political deal to avoid a runoff, saying he would run in a second round if needed.
"We are in a 100-minute game and we've only done 10 minutes," he said. He noted that he and Abdullah are only a little over 21,000 votes apart and millions of votes remain to be counted.
The chairman of the Independent Election Commission, Ahmad Yousuf Nouristani, also said it was too soon to predict the outcome. "Maybe today one candidate looks strong. Tomorrow, maybe another will pull ahead," he said.
Final results are to be declared in mid-May once complaints of fraud are fully investigated.
The man who replaces Karzai, the only president Afghans have known since the U.S.-led invasion ousted the Taliban in 2001, faces a huge task in fighting the insurgents and overseeing the withdrawal of the last foreign combat troops by the end of this year.
Both front-runners support women's rights and differ largely on domestic issues such as relations with Pakistan and peace talks with the Taliban. They also have promised to sign a bilateral security agreement with the United States that would allow thousands of foreign troops to remain in the country in a training and advisory role after 2014.
Abdullah has said he would sign the security pact within a month of taking office.
"You'll not find me one of those candidates that during the election I say one thing and after another," he said Sunday. "My position in regards to the (pact) is that it should be signed as soon as possible and is long overdue already."
Ahmadzai, who chaired a commission overseeing the transition of responsibility for security from the U.S.-led coalition to Afghan forces, also has said he would sign the agreement shortly after taking office.
The 64-year-old academic also ran for president in 2009 but received just 3 percent of the vote. He has gained popularity among many Afghans looking for a candidate who is not tied to the country's troubled past and promises change.
Ahmadzai also has hedged his bets, choosing as his main running mate a powerful warlord, Gen. Rashid Dostum, who brings the support of many of Afghanistan's Uzbeks, roughly 9 percent of the country's 32 million people.
Rassoul issued a statement later Sunday saying he had met with the other five candidates.
"Our team has confidence in the electoral process while calling on the electoral commissions to transparently distinguish valid votes from fraudulent ballots and not compromise the actual votes of the Afghan people," Rassoul said.
The Taliban, which launched dozens of attacks in the run-up to the April 5 balloting but failed to squash the enthusiasm of millions of men and women who lined up for hours to cast ballots, rejected Sunday's results.
"These elections and their results are not legitimate. The country is occupied and there is fighting in the big cities. The winner and the loser are both criminals," Taliban spokesman Qari Yousef Ahmadi said.
The results released Sunday were from 26 of Afghanistan's 34 provinces and represented just over 500,000 ballots, Nouristani said. Abdullah, who came in second to Karzai in the disputed 2009 election, had 212,312 votes. Ghani had 190,561 and Rassoul trailed with 49,821 votes. Numbers for the other five candidates were not announced.
"The results match what I think the mood has been over the last few days," said Martine van Bijlert, co-director of the independent research group Afghanistan Analysts Network. "We've got two front-runners and the two of them are clearly ahead of all the other candidates."
Nearly 1,900 complaints of fraud have been filed, 870 of which are serious enough to potentially affect results, said Mohammed Nadir Mohseni, the spokesman of the complaints committee of the Independent Elections Commission.
Widespread allegations of fraud marred the 2009 vote and led to a third of the ballots for Karzai being disqualified, depriving him of the majority needed to avoid a runoff. Abdullah quit before a second round could be held, saying he did not believe it would be fair.
Associated Press writer Amir Shah contributed to this report.
Kim Gamel can be followed on Twitter at www.twitter.com/kimgamel.