“There is a clear intention to boycott the will of the people,” she said, without providing detailed evidence.
She made the statement after her left-wing rival, Pedro Castillo, backed away from her during the vote count.
With 97.3% of the votes counted, Mr Castillo had a razor lead of less than 0.5 percentage points.
In a press conference, Ms. Fujimori said that there was a “strategy of Peru Libre [Free Peru, the party of Mr Castillo] to distort and delay results that reflect popular will. “
Peru Libre denied the accusation, writing on Twitter that “he has never resorted to electoral fraud, on the contrary, we have always been victims of it.”
After accusing her rival’s party of fraud, Ms. Fujimori made a more optimistic note, saying the masses could still turn around after the votes of Peruvians abroad were counted.
The votes, which are yet to be counted, come mainly from remote rural areas in Peru and from Peruvians who vote at consulates around the world.
Mr. Castillo has strong support in rural areas, but Ms. Fujimori has done well with Peruvians living outside her homeland, making the end result extremely difficult to predict.
Whoever wins will have the difficult task of uniting a polarized nation that has seen its economy fall apart as it has been hit hard by the coronavirus pandemic.
Following a change in the methodology for registering Covid deaths, Peru has become the country with the highest coronavirus deaths per capita in the world.
In an effort to attract those affected by the pandemic, Ms. Fujimori said that if elected, her government would give 10,000 feet ($ 2,600; $ 1,830) to families who lost a relative of Covid-19.
But many Peruvians fear that if she wins, the country’s “old guard” could return to power. Ms. Fujimori is the daughter of former President Alberto Fujimori, who is serving a 25-year sentence for crimes, including corruption and human rights abuses.
She said that if elected, she would pardon her father, who remains a deeply divisive figure – praised by some for his fight against the Brilliant Road guerrillas and denounced by others for abuses committed during his rule, including coercive sterilization of indigenous women.
Meanwhile, there are those who fear that Pedro Castillo, a primary school teacher who was little known before winning the first round of elections, could destabilize the country’s economy.
The left-wing unionist has campaigned to help the poor by imposing higher taxes on powerful mining companies in the copper-producing country.
On Monday, the Peruvian currency fell to a record low as Mr Castillo overtook Ms Fujimori in the vote count.
He also said he wanted to rewrite the constitution “to end all inequalities” and vowed to make human rights a priority.
Whoever becomes president will also have to fight a congress in which 10 parties are fighting for control and which has ousted presidents in the past.
In the last four years, Peru has had four leaders: one resigned over a vote-buying scandal, another was impeachment, a third stepped down after less than a week in office, and a fourth is the current interim president.