After changing the methodology of how it registers Covid deaths, Peru became the country with the highest per capita death rate from coronavirus in the world.
Many Peruvians hold a deep mistrust of politicians following two decades in which five former presidents have been investigated or prosecuted for corruption.
Meanwhile, Castillo, a former teachers' union leader, was a favorite among Peru's rural communities. "I believe that in Peru (voters) who were indecisive are no more; those people who were inclined (toward a political party) are no more".
But three hours later the pollsters released a quick count that showed Castillo in front with 50.2 per cent, bringing scenes of joy and celebrations to the northern Cajamarca region.
Mr Castillo has strong support in rural areas but Ms Fujimori has done well with Peruvians living outside their homeland, making the final result extremely hard to predict.
Polls in the runoff election began to open at 7 a.m. (1200 GMT), with long queues building up early at some centres in the capital Lima of people bundled up against the cold of the southern hemisphere late autumn.
"We're not going to know (the winner) until the last vote" is counted, political scientist Jessica Smith told AFP.
Corvetto urged fellow Peruvians to wait for official results to come in from the provinces.
Castillo, 51, had topped the first round of voting in April, when the pair both caused a surprise by reaching the second round, and he was also narrowly ahead in the latest opinion polls before Sunday's vote. The yield on Peru's 10-year dollar bonds rose 19 basis points to 2.85% with the price falling 1.7 cent to 99 cents on the dollar. The rest of the city has been plastered with pro-Castillo banners, without a single pro-Fujimori sign to be seen.
Trade unionist schoolteacher Castillo has pledged to nationalize vital industries, raise taxes, eliminate tax exemptions and increase state regulation.
Ms Fujimori herself spent several months in custody on corruption allegations she denies.
But his government collapsed amid accusations of grand-scale corruption and he is now serving a 25-year jail sentence for ordering the extrajudicial killings of suspected subversives, most of whom turned out to have nothing to do with Shining Path.
On her way to an election breakfast in Lima, Fujimori told journalists: "Keiko means hope". Fujimori also called for calm and said that whoever wins will have to seek to unite the country.
In a news conference, Ms Fujimori alleged that there had been a "strategy by Peru Libre [Free Peru, the party of Mr Castillo] to distort and delay the results which reflect the popular will".