Official results were due later Sunday in the race between the heavily favored Fernandez and 13 rivals. Fernandez's husband, President Nestor Kirchner, is credited with Argentina's rebound from a 2001 economic collapse, and much of her support is due to his popularity.
Her closest challengers, former lawmaker Elisa Carrio and former Economy Minister Roberto Lavagna, were trying to force her into a Nov. 25 runoff. She needed 40 percent of the vote, with a lead of more than 10 percent over her nearest rival, to win outright.
Five independent television networks and at least one private radio station reported their exit polling indicated Fernandez has easily won a first-round victory. Three of the television networks released their numbers, giving Fernandez between 42 and 46 percent of the vote, with advantages of between 19 and 23 percentage points over Carrio.
At Fernandez's campaign headquarters, supporters jumped up and down and embraced one another.
But no opposition candidates conceded defeat, and some said there had been unprecedented fraud. A Lavagna spokesman said the candidate would file a judicial complaint about a "systematic lack of ballots" marked with his name. Candidate Vilma Ripoll denounced "ballot stealing."
Electoral officials denied any irregularities, but a judge extended voting by an hour in the capital to make up for polling places that opened late.