KABUL — Afghanistan’s presidential challenger announced today he would not participate in next weekend’s runoff election because his demands for measures to prevent fraud were rejected. He stopped short of calling on his supporters to boycott the balloting.

“I will not participate in the November 7 election,” Abdullah Abdullah told supporters, because a “transparent election is not possible.”

Abdullah made no mention of agreeing to take part in any future unity government with President Hamid Karzai, which the U.S. and its international partners believe is the best hope for curbing the Taliban insurgency.

Asked by reporters if he was calling for a boycott, Abdullah said: “I have not made that call.”

Abdullah’s running mate, Homayoun Assefy, said it was up to the government’s Independent Election Commission to decide whether to hold the runoff next Saturday as scheduled.

Karzai campaign spokesman, Waheed Omar, said it was “very unfortunate” that Abdullah had withdrawn from the race but that the Saturday runoff should proceed without him.

“We believe that the elections have to go on, the process has to complete itself, the people of Afghanistan have to be given the right to vote,” Omar said.

In an emotional speech, Abdullah said that he did not believe a free and fair election was possible without changes in the leadership of the electoral commission, which ran the fraud-marred first-round ballot Aug. 20. Abdullah said the Afghan people should not accept results of a ballot run by the current group.

He told supporters that Karzai’s government had not been legitimate since its mandate expired last May. The Supreme Court, appointed by Karzai, extended his mandate after the election was put off from last spring until August.

A runoff was ordered after U.N. auditors threw out nearly a third of Karzai’s votes in the first round ballot, dropping him below the 50 percent threshold for victory in the 36-candidate field.

Abdullah denounced the Karzai-appointed election commission for fraud in the first round ballot and said his proposals for changes in the electoral body had all been rejected.

“In one hour, all my conditions could have been implemented,” he said. “Unfortunately until the last moment we were waiting, but we heard they rejected our appeals. And the transparency of the elections was not possible.”

He said the people of Afghanistan “have the right” to a free and fair election but the last ballot “was a failure.”