Afghan President Hamid Karzai inspects an honour gaurd with UN Secretary General Ban Ki-moon in Kabul on Monday. (Ahmad Masood/Reuters)

Afghan President Hamid Karzai was declared the victor of the war-torn country's presidential election as a run-off vote was cancelled after his opponent withdrew.

Independent Election Commission chairman Azizullah Lodin declared Karzai the victor during a news conference in Kabul on Monday. "He was the winner of the first round and the only candidate in the second round," Lodin said.

The commission came to its decision after examining the Afghan constitution and considering what is best for the Afghan people, Lodin said.

Electoral officers had earlier expressed concerns about voter security in the second round. The Taliban had threatened more violence if the vote was held, complicating the organization and security for the balloting.

Afghanistan's election commission cancelled the Nov. 7 vote hours after UN Secretary General Ban Ki-moon met with Karzai and former presidential candidate Abdullah Abdullah.

Decision won't solve problems: Abdullah campaign

"The announcement that was made by the electoral commission today will not solve the problems of Afghanistan, and it doesn't have any basis in law," said Abdullah spokesman Fazel Sancharaki.

Independent Election Commission chairman Azizullah Lodin listens to questions at a press conference in Kabul on Monday. (Gemunu Amarasinghe/Associated Press)

Sancharaki said Abdullah would comment further on the commission's decision on Tuesday but declined to say if the campaign would challenge the outcome.

U.S. President Barack Obama phoned Karzai to offer praise, but also to encourage an end to corruption in the country.

"I emphasized that this has to be a point in time in which we begin to write a new chapter," Obama said in describing his call to the Afghan leader.

Karzai, who had also said he wanted the run-off to go ahead, had effectively already secured a second term when Abdullah, his only challenger, dropped out of the race on Sunday. Abdullah alleged the vote would not be transparent or fair.

Abdullah had complained that Karzai turned down his demands for changes in the Independent Election Commission and other measures that Abdullah said would prevent massive fraud in the second round of balloting.

An investigation by the UN-backed election panel last month invalidated ballots from thousands of polling stations in the first round of voting due to fraud that was characterized by rampant ballot-box stuffing. The elimination of the ballots dropped Karzai below the 50 per cent threshold needed to avoid a second round of voting.

Karzai has led Afghanistan since U.S. forces invaded to oust the Taliban in 2001. He won elections in 2004 and his latest victory will give him another five-year mandate.

Ban congratulated Karzai on the results and praised the cancellation of the run-off vote. The United States and the United Kingdom have also issued statements congratulating Karzai on winning a second term.

'Difficult election process'

"This has been a difficult election process for Afghanistan, and lessons must be learned," said Ban. "Afghanistan now faces significant challenges and the new president must move swiftly to form a government that is able to command the support of both the Afghan people and the international community."

UN Secretary General Ban Ki-moon shakes hands with former presidential challenger Abdullah Abdullah in Kabul on Monday. (Altaf Qadri/Associated Press)

The presidential election was seen as crucial to strengthening the credibility of the Afghan government in the eyes of its citizens and allies.

But Abdullah's withdrawal may still throw the legitimacy of the new government into question. It will also likely be seen as a blow for Obama who is considering whether to send more troops to the war-torn country, according to analysts.

Analysts said cancelling the run-off vote was likely the favoured resolution by the international community and the focus will now likely be on encouraging Karzai to put together a government that will root out corruption.

"Karzai has lost his legitimacy, he is a very weak president and he cannot govern without reaching out to Dr. Abdullah," Kabul-based political analyst Haroun Mir told Reuters.

With files from The Associated Press