A "transparent election is not possible" because election officials hired by Karzai are not impartial, Abdullah Abdullah told a Kabul news conference.
Karzai had agreed to the runoff after an independent panel of election monitors rejected nearly one-third of his votes from the first round of voting in August.
"I want this to be an example for the future so that no one again tries to use fraud to abuse the rights of the Afghan people," CNN quoted Abdullah as saying Sunday.
Karzai spokesman Waheed Omar called Abdullah's decision "very unfortunate." The Nov. 7 vote, however, should go ahead to show the election process in Afghanistan works, Omar told CNN.
U.S. Secretary of State Hillary Clinton Saturday said the legitimacy of the election would not be affected if Abdullah withdrew, The New York Times reported Sunday.
"When President Karzai accepted the second round -- without knowing what the consequences and outcome would be -- that bestowed legitimacy from that moment forward," Clinton said in Jerusalem.