The practice is part of the robust political machine that has strengthened the Colorado Party’s grip on Paraguay, which it has controlled for 71 of the past 76 years, including four decades of military dictatorship.

The Colorado presidential candidate, Santiago Peña, won by 460,000 votes, with 43 percent of the total. (Paraguay has fewer than 80,000 Indigenous adults, according to estimates.) Mr. Peña is the political protégé of Horacio Cartes, a former president and the current party chairman, who was sanctioned this year by the U.S. government over accusations that he had bribed his way to power.

The second- and third-place candidates have suggested that Mr. Peña’s victory was rigged, but have not presented clear evidence. The third-place candidate, whose supporters have blocked highways in protest, has been jailed on accusations of attempting to obstruct elections.

In an interview before the election, Mr. Peña said that if vote buying happens, it would not swing races.

“The vote-buying argument doesn’t really have much evidence,” he said. “It has never been possible to demonstrate a massive purchase scheme. If 2.5 to 3 million people vote, how many votes would we have to buy?”