Watch "Amanpour" for more about why Abdullah dropped out of the Afghan presidential race on CNN at 2 p.m. ET Sunday and on CNN International at 3 p.m. ET.

Kabul, Afghanistan (CNN) -- Afghan politician Abdullah Abdullah withdrew Sunday from the upcoming runoff election, saying he believes that the second round would be as fraudulent as the first.

"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," Abdullah said.

Abdullah's withdrawal was not expected to change plans for the November 7 runoff election and ensures the re-election of President Hamid Karzai if the voting proceeds as scheduled.

Azizullah Lodin, the president of Afghanistan's Independent Election Commission, said Sunday that an announcement on holding the runoff would be made Monday.

In a statement, Karzai's campaign spokesman said the president would respect whatever decision the commission's makes about whether to hold the election.

Karzai initially had claimed victory after the first ballot in August, but a U.N.-backed panel of election monitors threw out nearly a third of his votes, citing fraud. When that left Karzai short of a majority, he agreed to the runoff, set for Saturday.

Abdullah had called for the resignations of top election officials and politicians to avert electoral fraud in the runoff. He argued that the commissioners, who are hired by Karzai, cannot be impartial.

But that request was not met, he said Sunday, and he did not believe that a transparent election was possible.

"I spoke to Karzai about the Independent Election Commission abusing its powers, but he denied it and refused to dismiss them," he said.

The election commission, which denied any wrongdoing, slammed the candidate's accusations.

Unless Abdullah has proof of such bias, he is in no position to ask any Afghan official to step down, Lodin said.

In a statement Sunday, the U.S. Embassy said "the United States remains committed to supporting the Afghan people in carrying out a constitutional electoral process."

"The United States looks forward to working with the next Afghan administration and with the Afghan people and our partners in the international community to support Afghanistan's progress towards peace and stability," the embassy statement said.

Also Sunday, the U.N. Assistance Mission in Afghanistan said "the next step must be to bring this electoral process to a conclusion in a legal and timely manner."

In a statement, a representative for the U.N. group credited Abdullah with having "acted in a statesmanlike and dignified way and put forward a number of constructive proposals for reform which I hope will be part of the political agenda for Afghanistan's future."

British Prime Minister Gordon Brown said in a statement that he was "confident that Afghanistan's leaders will support the remaining steps of the democratic process."

"We hope to see an Afghan government emerge that responds to the will of the people, that reaches out to all parts of Afghan society and that is ready to take strong action to meet the challenges that Afghanistan faces," Brown's statement said. "We are ready to work in close partnership with such a government."

Abdullah did not address any plans to be part of a possible coalition government, but he did not rule it out either.

"I will not focus on that," he told CNN's Christiane Amanpour on Sunday, adding that his goal was to contribute to a better Afghanistan in whatever capacity he found himself. He urged his supporters to remain calm and not take to the streets in protest.

"I will be available to serve this country," he said. "I am in consultations with my followers. ... A movement for change will be there, and I will lead that movement."

A spokesman for Karzai said it was "very unfortunate" that Abdullah had decided to boycott the runoff. However, the vote should be held anyway, for the sake of Afghans, Waheed Omar said.

"The process has to continue. ... The people of Afghanistan have to be given the right to vote," Omar said.

Abdullah rejected any assertion that his withdrawal meant people should boycott the vote.

"Absolutely not," he said. "I will not infringe into the rights of our citizens. This is my decision, and the people of Afghanistan will judge it for themselves and make decisions for themselves."

He also said he opposed any violence from his supporters, adding that he "called upon them to stay calm, patient."

Asked about how Karzai can plan to go ahead with a one-person runoff, Abdullah responded, "You can ask him."

"But I'm not getting into the legal implications and constitutional implications of my decision," Abdullah said.

Zalmay Khalilzad, a former U.S. ambassador to Afghanistan, predicted last week in an interview to air at 2 p.m. ET Sunday on CNN's "Amanpour" that Abdullah would quit the race.

"First, he doesn't have much money left," he said. "Second, I think that he thinks that given the situation, he's likely to lose, and maybe he'll get less votes than he did in the first round, so that would be embarrassing."

At a news conference in Jerusalem on Saturday, U.S. Secretary of State Hillary Clinton said the legitimacy of the race would not be affected in the event of Abdullah's withdrawal.

On Sunday, Clinton said in a statement, "We hope that [Abdullah] will continue to stay engaged in the national dialogue and work on behalf of the security and prosperity of the people of Afghanistan."

"It is now a matter for the Afghan authorities to decide on a way ahead that brings this electoral process to a conclusion in line with the Afghan constitution," Clinton's statement added. "We will support the next president and the people of Afghanistan, who seek and deserve a better future."

Abdullah's announcement comes after Gen. Stanley McChrystal, commander of forces in Afghanistan, has asked for additional troops to fight the Taliban insurgency, which has escalated its violence recently.

The militants have vowed to disrupt the election and claimed responsibility for a Wednesday attack that killed five United Nations workers, two of whom were helping with the vote. Nine others were wounded after attackers stormed a guest house in Kabul. In response, the United Nations ordered nonessential personnel out of the country.

A record number of U.S. troops were killed in Afghanistan in October: at least 58.

Abdullah said issues such as security and the possibility of more attacks were factors in his decision.

"This is not in exchange for anything from anybody," he said. "My decision was purely to give the people of Afghanistan a chance to move on."

CNN's Thomas Evans contributed to this report.