After President Hamid Karzai snubbed a series of demands promoted as a bid to avoid a repeat of massive first-round fraud, Abdullah said he saw no point in standing in the second round, while stopping short of calling for a boycott.
But Karzai´s camp insisted the contest should still go ahead, with analysts saying a one-horse race could still take place on November 7 even if turnout is likely to be well below the 38 percent recorded last time.
"The decision which I am going to announce was not an easy one. It was a decision that I have taken after wide-ranging consultations, with the people of Afghanistan, my supporters and influential leaders," Abdullah told supporters.
"In protest against the misconduct of the government and the Independent Election Commission (IEC), I will not participate in the election," he added in an address in Kabul.
During his lengthy speech, the former foreign minister launched a scathing attack on the "eight years of lost opportunities" during Karzai´s rule, dimming prospects that the rivals could yet agree on a form of power-sharing.
And in a later press conference, he denied he had cut any deals.
"This is my decision. This decision has not been made in exchange for anything from anybody," he said.
Following the widespread fraud in the first round on August 20, Abdullah demanded that Karzai sack the head of the IEC, Azizullah Ludin, and suspend four ministers who campaigned for the incumbent.
Abdullah´s camp set a deadline of Saturday for Karzai to bow to his demands, saying he would not take part in a contest that will not be free and fair.
But his demands received short shrift, with the IEC saying Ludin can only be dismissed by the supreme court while Karzai said Abdullah has no right to interfere in ministerial positions.
Abdullah said he had no faith in the prospect of a free and fair election next Saturday while the head of the commission, who was appointed by Karzai, remained in place.
"The election commission was not independent. If it was, the people would not face such a problem. It was their job to ensure a credible election and we saw what happened," he said.
Abdullah urged supporters to refrain from protests which could enflame the situation and said it was up to them whether they choose to vote next Saturday.
"I leave the choice to my followers and sympathisers," Abdullah said, when asked whether he was instructing his followers to refrain from voting.
After the announcement, Karzai´s chief campaign spokesman said the run-off should still take place.
"We believe that the election has to go on, the process must complete itself," Waheed Omar said.
Kabul-based analyst Haroun Mir predicted the election would still take place in line with the constitution but said Karzai risked losing his legitimacy.
"The Independent Election Commission has decided to hold the run-off and President Karzai will participate," Mir, head of Afghanistan´s Centre for Research and Policy Studies, told AFP.
"If voter turnout is very, very low, below 20 percent, then even though he will be declared winner he will lack legitimacy in the eyes of Afghans and the opposition."
Asked whether the outcome of a run-off with only one candidate would result in a legitimate government, US Secretary of State Hillary Clinton said Saturday that such situations are "not unprecedented."
"We see that happen in our own country where, for whatever combination of reasons, one of the candidates decides not to go forward. I don´t think it has anything to do with the legitimacy of the election," she said.
Karzai´s share of the vote in the first round fell to 49.67 percent after a UN-backed watchdog deemed around a quarter of all votes cast to be fraudulent.
Insistent that the fraud had been overstated, Karzai only agreed to a second round under extensive diplomatic pressure from Washington, highlighted when he made the run-off announcement standing alongside US Senator John Kerry.
Abdullah won just over 30 percent in the first round and would have had a mountain to climb if he were to have overhauled Karzai in the run-off.
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