The New York Times quoted tribal leaders in Shorabak District in Kandahar Province, as saying that aides to Karzai's brother Ahmed Wali detained the governor of Shorabak, Delaga Bariz, and shut down all of the district's 45 polling sites on election day.
They said the ballot boxes were taken to Shorabak's district headquarters, where, local police officers stuffed them with thousands of ballots.
At the end of the day, 23,900 ballots were shipped to Kabul, Bariz said, with every one marked for President Karzai.
"Not a single person in Shorabak District cast a ballot - not a single person," Bariz said in an interview in Kabul, where he and a group of tribal elders came to file a complaint.
The accusations by Bariz, and several other tribal leaders from Shorabak, are the most serious allegations so far that have been publicized against Karzai's electoral machine, which faces a deluge of fraud complaints from around the country.
The Afghan Electoral Complaints Commission said Tuesday, Sep 1, that the number of complaints about vote stealing and other forms of fraud had reached 2,615.
In Kandahar Province, where Karzai's family is in control, allegations of a type similar to those made in Shorabak have been made in many of the province's 17 districts.
Early election returns show that Karzai has managed to capture nearly 48,000 votes, compared with only 3,000 for Abdullah, his nearest challenger.
Slightly less than half of all ballots have been counted. Karzai leads with about 46 percent of the vote, compared with 33 percent for Abdullah.
However, Karzai and his team denied the allegations.