Afghan President Hamid Karzai's main challenger has agreed to participate in a runoff election that will send Afghan voters back to the polls in the first week of November.

Abdullah Abdullah, the country's foreign minister, agreed to take part in the runoff on Wednesday, the day after the incumbent Afghan president gave his blessing for a second vote.

Abdullah said he called Karzai to thank him for agreeing to take part in a runoff, which will leave Afghan officials scrambling to arrange a Nov. 7 vote that comes against a backdrop of an approaching winter season and an ever-present Taliban threat.

He called on Afghan and U.S. forces to provide adequate security for people who are risking a lot to vote for a second time.

Voters "are taking a risk in some parts of the country and they should be confident that that risk is worthwhile," Abdullah said Wednesday.

"I would like to see that our people are participating without an environment and atmosphere of fear and intimidation."

The runoff was called after the UN-backed Electoral Complaints Commission revealed that it had thrown out nearly one-third of Karzai's votes from the Aug. 20 election, because of fraud. As a result, Afghan officials have fired 200 district election chiefs, following complaints from within their regions.

The country's Independent Election Commission must also finalize a list of polling stations for the Nov. 7 vote.

Preliminary results showed Karzai with about 54 per cent of the overall vote from the August election. When hundreds of thousands of his votes were later thrown out, he remained in the lead, but his support dropped below the 50 per cent mark.

Under Afghan law, a runoff between the top two candidates must take place if no candidate manages to garner more than half of the overall votes in a presidential election.

Meeting expectations will pose a "huge challenge" in the second election, said UN Secretary-General Ban Ki-moon.

Aleem Siddique, the UN spokesperson in Kabul, said the world body had earmarked US$20 million to support the poll.

Nipa Banerjee, a University of Ottawa professor who observed the August election, said she believes many Afghans will decline to take part in a second vote.

"I think we have lost trust in the process of democracy, to a certain extent, and therefore, I think the turnout is going to be lower this time," Banerjee told CTV's Canada AM during an interview from Ottawa on Wednesday morning.

Kabul-based Human Rights Watch researcher Rachel Reid questioned how Afghan officials would improve conditions for women to vote in the Nov. 7 runoff.

"It is hard to see how a second round can be credible unless women's security and access to the polls is dramatically improved," Reid said.

With files from The Associated Press