KABUL — A U.N.-backed fraud commission threw out votes today from 83 polling stations and ordered recounts at hundreds of others in three provinces that form Afghan President Hamid Karzai's political base, reducing his chances of avoiding a runoff.

It was the first time the commission has flexed its muscles in the aftermath of an Aug. 20 presidential election marred by allegations of ballot stuffing, phantom polling stations and turnout at some polls that exceeded 100 percent of registered voters.

In an interview with The Associated Press, Karzai's chief challenger, former foreign minister Abdullah Abdullah, charged that the massive scale of what he called "state-engineered" fraud has become clear only as the numbers have trickled out over the past three weeks.