Interim leader Hamid Karzai (search) is certain to win Afghanistan's (search) landmark presidential election, his campaign spokesman said Tuesday, after early returns gave him a commanding advantage.

With one-quarter of the votes from the Oct. 9 ballot counted, Karzai has captured 61.8 percent. His closest challenger, former Education Minister Yunus Qanooni (search), trails with 18.2 percent.

"We think we are secure now," Karzai's spokesman, Hamed Elmi, told The Associated Press. "When they announce it formally, then we will celebrate."

Elmi said that Karzai was "quite pleased" with the results so far and his campaign staff were "100 percent" sure that the U.S.-backed incumbent would win the majority of the 8 million votes needed to avoid a run-off.

Qanooni has so far refused to concede defeat and claimed on Monday that only fraud has given Karzai the lead in the race to become the country's first ever popularly elected leader.

On Tuesday, the running-mate of ethnic Uzbek warlord Abdul Rashid Dostum, currently third in the vote count, conceded defeat.

Chafiga Habibi, vice-presidential candidate on Dostum's ticket, told AP, "I think Karzai is going to win because he's a long way ahead in the results, and we can't ignore this reality."

Election officials say they will not call the result until the winner is certain, but have also said that the tallies are unlikely to change much once 20 percent of the votes have been counted — a point reached on Monday.

Despite poor weather and Taliban threats of more attacks, an estimated 8 million Afghans cast their ballots in a democratic experiment supposed to cement the country's re-emergence since the fall of the Taliban in 2001.

In a reminder of the country's continuing insecurity, a doctor who helped to organize the election was killed along with four other civilians when an explosion tore through their vehicle on Monday, police said.

Election officials said it was unclear if the vehicle was deliberately targeted by the blast in southeastern Paktika province. But the local police chief blamed "the enemies of Afghanistan" — shorthand for Taliban militants who have threatened to disrupt the elections.

Karzai, Afghanistan's interim leader since the Taliban's ouster in 2001, is seen by many Afghans as a bridge to its international backers and a leader untainted by its bloody past.

Still, many Afghans are impatient at the slow pace of reconstruction, and minorities are wary of his strong support in the Pashtun-dominated south — from where the Taliban also drew their main strength.

Qanooni, an ethnic Tajik, said on Monday that he believed he would be leading in the vote-count if the ballot had been fair.

He alleged that ballot boxes had been stuffed with votes in favor of Karzai in at least four provinces — but election officials say there is no evidence of this.

Karzai's rivals have lodged dozens of complaints with a panel of foreign experts set up to head off their threat to boycott the results.