Just a week before this countrys presidential election, the leaders of a southern Afghan tribe called Bariz gathered to make a bold decision: they would abandon the incumbent and local favorite, Hamid Karzai, and endorse his challenger, Abdullah Abdullah. Mr. Abdullah flew to the southern city of Kandahar to receive the tribes endorsement. The leaders of the tribe, who live in a district called Shorabak, prepared to deliver a local landslide. But it never happened, the tribal leaders said. Instead, aides to Mr. Karzais brother Ahmed Wali the leader of the Kandahar provincial council and the most powerful man in southern Afghanistan detained the governor of Shorabak, Delaga Bariz, and shut down all of the districts 45 polling sites on election day. The ballot boxes were taken to Shorabaks district headquarters, where, Mr. Bariz and other tribal leaders said, local police officers stuffed them with thousands of ballots. At the end of the day, 23,900 ballots were shipped to Kabul, Mr. Bariz said, with every one marked for President Karzai. Not a single person in Shorabak District cast a ballot not a single person, Mr. Bariz said in an interview here in the capital, where he and a group of tribal elders came to file a complaint. Mr. Karzais people stuffed all the ballot boxes. The accusations by Mr. Bariz, and several other tribal leaders from Shorabak, are the most serious allegations so far that have been publicized against Mr. Karzais electoral machine, which faces a deluge of fraud complaints from around the country. The Afghan Electoral Complaints Commission said Tuesday that the number of complaints about vote stealing and other forms of fraud had reached 2,615. Mr. Karzais campaign is accused of forging ballots, stealing votes and preventing people from going to the polls. In Kandahar Province, where Mr. Karzais family is in control, allegations of a type similar to those made in Shorabak have been made in many of the provinces 17 districts. Early election returns show that Mr. Karzai has managed to capture nearly 48,000 votes, compared with only 3,000 for Mr. Abdullah, his nearest challenger. Slightly less than half of all ballots have been counted. Mr. Karzai leads with about 46 percent of the vote, compared with 33 percent for Mr. Abdullah. Mr. Karzai and his aides deny any sort of fraud, and they have hunkered down in the presidential palace to await the final results. But the allegations are casting a cloud over his re-election campaign, raising the prospect that even if he wins his presidency could be seriously tainted. At the same time, the allegations are increasing the pressure on American officials to ensure that the accusations of fraud are properly investigated. An election widely perceived as having been stolen could deal a serious setback to the Obama administration, which has committed itself to prevailing here in the nearly eight-year-old war against the Taliban and Al Qaeda. Allegations like those described by Mr. Bariz are throwing the basic integrity of the election into question. Much of the story told by Mr. Bariz and the other tribal elders was impossible to verify. But it appeared credible. All three men spoke in great detail. And all of them were willing to be publicly named and to have their photographs taken. As recently as 10 months ago, Mr. Bariz said, he had considered himself an ally of President Karzai. He had been nominated by a group of Bariz elders to be the governor of the Shorabak District, a desolate stretch of sand and scrub that sits on the countrys southwestern border with Pakistan. Mr. Barizs nomination was ratified by Tooryalai Wesa, the governor of Kandahar Province, who was appointed by President Karzai. But as election day neared, Mr. Bariz and other leaders in his tribe said they could not bring themselves to support Mr. Karzai for another five-year term. The reason, he said, was that Mr. Karzais government had done so little good. There are no clinics, no schools, no roads, no water dams nothing, Mr. Bariz said. We decided to support someone who would unify the country. The leaders of the Bariz tribe picked Mr. Abdullah, a former foreign minister. In theory, the decision by the elders sealed Mr. Abdullahs victory in Shorabak: nearly everyone in Shorabak belongs to the Bariz tribe. As is common in many such societies, tribal leaders in Afghanistan often negotiate with politicians to deliver the votes of their tribe. Mr. Abdullahs campaign manager in southern Afghanistan, Esmatullah, who like many Afghans uses only one name, said the candidate met a large group of Bariz tribal elders in Kandahar on Aug. 12 to receive their endorsement. It was a joyous affair, Mr. Esmatullah said, for which even women turned out. But not everyone who wanted to come to the endorsement ceremony was able to make it. The police were blocking the roads, Mr. Esmatullah said. The next day, Mr. Bariz said, officials in Kandahar were furious. One of Kandahars senior officials, Mohammed Anas, ordered Mr. Bariz not to return to his home in Shorabak. Mr. Anas said he had no choice. When I asked him why he wouldnt let me go home, he said, 'Because your whole tribe is going to vote for Dr. Abdullah, Mr. Bariz said. Mr. Bariz did not speak to Ahmed Wali Karzai, the presidents younger brother, only to more junior officials like Mr. Anas. But few decisions of any import are believed to be taken in Kandahar without the approval of Ahmed Wali Karzai. On the streets, his nickname is The King of the South. Last year, for instance, Ahmed Wali Karzai was widely seen as having replaced the governor, Rahmatullah Raufi, when he fell out of favor. Attempts to contact Ahmed Wali Karzai were unsuccessful. When election day finally came, the ballots were never delivered to the polling centers in Shorabak, said two Bariz tribal leaders who were charged with overseeing the sites. Instead of going to the polling places, all the ballots and ballot boxes were delivered to the district governments headquarters. That place, the tribal leaders said, had been commandeered by the Afghan Border Police. The ballots were never delivered, said Abdul Quyoum, a farmer from the village of Karaze, where one of the polling sites was supposed to be. I waited all day. Mr. Quyoum was one of two tribal elders from Shorabak who traveled to Kabul with Mr. Bariz. The other was Fazul Mohammed, who told a nearly identical story. When the ballots were not delivered to the polling site, Mr. Mohammed said, he walked to the district government headquarters to see what was wrong. The building, he said, was being guarded by officers of the Afghan Border Police. As an election official, Mr. Mohammed said, he was allowed to go inside. The border police were stuffing the ballots, hundreds of them, into the boxes, he said. And there were other people who were counting the ballots and keeping the records. Mr. Mohammed said he protested but was told to leave. Later, he said, he was told that a total of 23,900 ballots had been filled out, all in Mr. Karzais name. Dr. Abdullah did not receive a single vote, he said. Mr. Bariz, the governor, said he had not returned to Shorabak. I dont think I am going to be governor much longer, he said.