Conservative Keiko Fujimori inched into the lead late on Wednesday night, boosted by overseas ballots that gave her 50.002 per cent of the vote to leftist Roberto Sanchez's 49.998 per cent, a difference of about 600 votes out of 18 million.
More than 98 per cent of the vote has been counted according to Peru's ONPE electoral authority, but most of those remaining are contested ballots.
Only nine of the more than 90,000 polling stations remain to be counted while about 1600 - representing around 400,000 votes - have been sent for review to Peru's National Elections Jury (JNE).
The review is expected to take weeks. Most of the contested ballots are from the Lima metropolitan region, Fujimori's stronghold.
The two candidates have been neck-and-neck throughout the count, with Fujimori leading exit polls and Sanchez being slightly ahead in the Ipsos quick count, which has accurately predicted previous races.
Fujimori and Sanchez had called for calm and patience throughout the count, but Sanchez - who overtook Fujimori on Monday, buoyed by rural votes - began to harden his tone on Wednesday and called for a meeting with international observers to discuss "strange, unusual and questionable developments."
On Thursday, Sanchez said he had important meetings with observers from the European Union and Organisation of American States.
"Our people are vigilant, the vote and democracy must be respected," Sanchez said on X when he announced the meetings took place.
Aside from the contested ballots, Sanchez's team is suing to annul votes from 1750 polling stations in Peru, mostly in the Lima area, for irregularities and votes from 657 polling stations in the United States for being "transported irregularly".
This is Fujimori's fourth consecutive runoff after losing the last two by just fractions of a per cent. In 2021, Fujimori - the daughter of polarising former President Alberto Fujimori - lost to now-jailed President Pedro Castillo by around 45,000 votes.
Sanchez, who served as a minister under Castillo, has been his political heir in this race, donning the same signature cowboy hat and waiting for early results outside the prison where Castillo is being held.
Speaking to reporters outside her home on Thursday, Fujimori reiterated that she would wait for the final count to be announced before making any definitive statement on the race.
Fujimori, who made accusations of fraud in the 2021 election she lost, said that the 2026 race had been carried out transparently with thousands of poll watchers from both parties and international observers.
"I think that gives (the election) the credibility, tranquility and trust of the citizenry," Fujimori said.
