MOSCOW, October 31 (RIA Novosti) -- Abdullah Abdullah, the main challenger to President Hamid Karzai, threatened on Saturday to boycott a presidential run-off election planned for November 7.

Spokesmen for Abdullah said he had no option but to boycott the contest, because Karzai had refused to meet his demands to dismiss the nation's top election official and take other action to prevent fraud.

"We don't want to boycott, but Mr. Karzai has not accepted any conditions, so he left us with no other choice," a member of Abdullah's political team was quoted as saying.

A source in the Abdullah camp said it would announce whether their candidate would pull out of the run-off on Sunday.

U.S. Secretary of State Hillary Clinton has said the Afghan elections would be legitimate even if Abdullah boycotted the run poll.

Clinton said the withdrawal of a candidate would not be "unprecedented" and would not affect the legitimacy of the vote.

Last week, President Karzai gave in to international pressure and agreed to take part in the runoff against his nearest rival, former foreign minister Abdullah Abdullah, accepting the conclusion of the UN-backed Electoral Complaints Commission that the election was the subject of wide-scale fraud.

The investigation determined that no candidate had received over 50% of the vote, the threshold for automatic victory. Official results gave Karzai 54.6% of the vote in the first round.