The populist retired army officer said he has called on his backers to register as poll watchers "so votes are not stolen from us during the tabulation at the polling tables."
Polls indicate that Garcia, a dazzling orator whose 1985-90 government was marked by hyperinflation, food shortages and rampant corruption, leads Humala by 12 to 16 percentage points.
Humala said his suspicions are based largely on a comment by Lourdes Flores, who told reporters she felt she had "lost at the tabulation tables, not at the ballot box" when she narrowly finished third behind Garcia in the April 9 first-round election, the AP reports.
Humala twice brushed off the question of whether he would respect the election results if he loses. "I believe there is no reason to be pessimistic," he said. "We're going to win in the second round."
Garcia on Tuesday said that Humala was crying fraud because he knows he is going to lose and Enrique Mendoza, president of Peru's National Election Board, challenged Humala to present evidence that a fraud was in the works.
Mendoza said a decision had been made not to renew Venezuela's representative as a poll watcher for lack of impartiality, the newspaper El Comercio reported.
Peru has accused Venezuela of "persistent and flagrant" interference in the country's internal affairs because leftist Venezuelan President Hugo Chavez has repeatedly endorsed Humala. Peru also recalled its ambassador from Caracas.
The points of view of Biden and Putin do not coincide in the understanding that the relations should be built on a mutually beneficial basis and coincidence of interests