KABUL — President Hamid Karzai widened his lead over his main challenger in election returns released Saturday, creeping toward the 50 percent mark that would enable him to avoid a run-off in the divisive presidential contest.

Karzai's top challenger, Abdullah Abdullah, stepped up his fraud charges, raising doubts whether the former foreign minister's followers would accept the incumbent if he wins in the first round.

Accusations of fraud in the Aug. 20 vote have poured into the Electoral Complaint Commission, which must investigate the allegations before final results can be announced.

Fraud allegations from Abdullah and other presidential candidates as well as low turnout in the violent south could strip the election of legitimacy, not only among Afghans but also among the United States and its international partners that have staked their Afghan policies on support for a credible government.

A widely accepted Afghan government is one of the pillars of President Barack Obama's strategy to turn the tide of the Taliban insurgency. The election controversy has boiled over at a time of rising U.S. and NATO casualties, undermining support for the war in the U.S., Britain and other countries with troops here.

British Prime Minister Gordon Brown paid a surprise visit Saturday to British troops in southern Afghanistan's Helmand province, hoping to counter critics who accuse his government of failing to support Britain's mission here. A British Marine was killed by a bomb in Helmand on the day of the prime minister's visit, the Ministry of Defense said in London.

Figures released Saturday show Karzai with 46.2 percent of the votes against Abdullah's 31.4 percent. The results are based on 35 percent of the country's polling stations, meaning the percentages could still change dramatically.

Few results have been announced from northern Balkh province, where Abdullah was expected to run strong, and from some southern Pashto-speaking provinces where Karzai draws his support.

Karzai's aides appeared confident that the president would score a first-round victory and avoid a run-off, which would probably be held in October if needed.

By contrast, Abdullah has been stepping up his charges, telling Italy's RAI television that Karzai was responsible for "state-crafted, massive election fraud."

Abdullah's drumbeat of allegations appear aimed in part at the United States and its allies, which would face some hard choices if a substantial number of fraud complaints are found to be true.

"If we allow he who robbed the votes of this country to move forward, we would give the Afghan people a future that they do not want to see, and I think this goes also for the international community," Abdullah said in the Italian interview.

Abdullah said he would keep his protests "within the confines of the law."

"But the fact is that the foundations of this country have been damaged by this fraud, throwing it open to all kinds of consequences, including instability," he added. "It is true that the Taliban are the first threat to this country, but an illegitimate government would be the second."

International officials — including Obama, the top U.N. official in Afghanistan and the European Commission — were quick to congratulate Afghans for pulling off the vote in the face of Taliban threats and violence.

But the massive fraud allegations that have surfaced since then have cast a dark shadow over the process, and some officials are withholding judgment on whether the election was credible.

The U.S.-funded International Republican Institute, which sent 29 international observers to monitor the balloting, urged authorities Saturday to ensure that "all complaints" be examined "in a prompt, fair and transparent manner" so that the results will be accepted by most Afghans.

Senior officials from 27 countries — including special U.S. envoy Richard Holbrooke — are to meet in Paris on Wednesday to discuss Afghanistan, and the disputed election is likely to dominate the agenda.

During his visit with British forces, Brown promised to provide more equipment to help his soldiers cope with Taliban roadside bombs, the major threat to NATO forces. More than 200 British soldiers have been killed in Afghanistan since the war began in 2001 — more than Britain lost in the Iraq conflict.

"Let me pay tribute to the courage, bravery, professionalism and patriotism of our forces," Brown told the troops at the British base in Lashkar Gah, capital of Helmand province. "I think our forces have shown extraordinary courage during this period. They know the reason why we are here and that is our security at home depends on a stable Afghanistan, no return of the Taliban, and no role for al-Qaida in the running of Afghanistan."

Brown called for speeding up the training of Afghan forces so they can play a bigger role in fighting the Taliban.

Britain has about 9,000 troops in Afghanistan, compared with more than 60,000 Americans. Obama ordered about 21,000 more U.S. troops to Afghanistan this year, raising the total international force to more than 100,000.