KABUL, Afghanistan — Under heavy international pressure, President Hamid Karzai conceded Tuesday that he fell short of a first-round victory in the nation’s disputed presidential election, and agreed to hold a runoff election with his top challenger on Nov. 7.

Flanked at a news conference in Kabul by Senator John Kerry, the head of the powerful Senate Foreign Relations Committee, and Kai Eide, the top United Nations official in Afghanistan, Mr. Karzai said he would accept the findings of an international audit that stripped him of nearly one-third of his votes in the first round, leaving him below the 50 percent threshold that would have allowed him to avoid a runoff and declare victory over his main rival, Abdullah Abdullah.

“I call upon this country to take this as an opportunity to move this country forward and participate in this new round of elections,” Mr. Karzai said, according to the English translation of his remarks, adding that he was grateful to the international community for its help.

Mr. Karzai called for continued international assistance in securing the country for the next round of voting. He did not express regret about the widespread fraud that a joint Afghan-international audit committee ruled Monday had occurred among the ballots marked in his name, but said the fraud would be investigated.

“Unfortunately, the election of Afghanistan was defamed,” Mr. Karzai said. “Any result that we were getting out of it was not able to bring legitimacy.”

In Washington, President Obama welcomed Mr. Karzai’s decision, saying he phoned the Afghan president to congratulate him and calling the runoff “an important step forward.”

“While this election could have remained unresolved to the detriment of the country, President Karzai’s constructive actions established an important precedent for Afghanistan’s new democracy,” Mr. Obama said in a statement. “The Afghan Constitution and laws are strengthened by President Karzai’s decision, which is in the best interests of the Afghan people.”

Mr. Karzai’s capitulation came after an all-out push by Obama administration officials and their European allies. But even though Mr. Karzai ended his strong resistance to a runoff, that will not completely resolve the country’s political crisis, officials say. It will be difficult to hold the new election as the Afghan winter approaches and under the perilous security challenges posed by the Taliban insurgency.

And there is still a chance that the two candidates could reach a power-sharing agreement, which would cancel the need for a second round. Mr. Abdullah has said repeatedly that after the results of the audit were announced, he might be open to other options. But it was not clear on Tuesday that Mr. Karzai was willing to offer them, denying at the news conference that there had been talks of a coalition government and saying he was looking forward to competing.

During a hastily arranged two-hour meeting with Mr. Kerry, Democrat of Massachusetts and a top foreign policy ally of Mr. Obama, and the United States ambassador, Lt. Gen. Karl W. Eikenberry, at the presidential palace in Kabul on Monday, Mr. Karzai, after initially hesitating, agreed to accept the findings, American and European officials said.

For Mr. Karzai, the decision to acquiesce to the demands of the international community puts him in the position of disappointing his followers, including people who showed up at the polls despite widespread threats from the Taliban to disrupt the elections.

“The dilemma for Karzai is that because of the tribal nature of Afghan society, if a constituency is angry at having a significant number of votes denied and reacts by withholding their vote in the next round, it could change the result,” said a senior administration official.

The United States, this official said, is sympathetic to Mr. Karzai’s concerns, but Hillary Rodham Clinton, the American secretary of state, urged him in calls over the last few days to be a “statesman” and accept the results.

A panel of experts appointed by the United Nations issued findings on Monday that showed that fraud was so pervasive that nearly a quarter of all votes were thrown out, and that Mr. Karzai had not won the Aug. 20 election outright, according to an analysis of the findings by The New York Times. Nearly a million of Mr. Karzai’s just over three million votes were discarded.

The findings are a defining moment for Mr. Karzai, who initially received 54 percent of the vote and who says he believes that he is the rightful winner. Mr. Karzai initially indicated that he might reject the committee’s findings, a Western official said.

Besides Mr. Kerry and General Eikenberry, Mr. Karzai was pushed hard by Prime Minister Gordon Brown of Britain during a telephone call on Monday, European officials said. The British ambassador to Afghanistan joined the American and United Nations officials at Mr. Karzai’s Tuesday news conference.

“A moment of great uncertainty has been transformed into a moment of great opportunity,” Mr. Kerry said at the event, praising Mr. Karzai’s decision to cooperate with the ruling of the audit committee, as mandated under Afghani election laws.

Mr. Karzai praised the bravery of Afghan voters and said he hoped for an even stronger turnout in the second round. “People, despite rockets, threats and intimidation, went to the ballot boxes and voted,” he said. “Our people, our youth, the national army and police sacrificed. And the people of Afghanistan were the winner of the election.”

Obama administration officials had tried to use Mr. Obama’s pending strategy review on Afghanistan as leverage on Mr. Karzai, telling their Afghan counterparts that Mr. Obama would not make a decision to add to the 68,000 American troops in the country until Mr. Karzai agreed to accept the election outcome.

On Sunday, Mr. Obama’s chief of staff, Rahm Emanuel, said in television interviews that the administration must first ensure it has a “credible” partner in the Afghan government before making a decision to send more troops. “That message has been conveyed to the highest levels of the Afghan leadership,” an American official said.