The result of Peru’s presidential election hangs in the balance after rightwing candidate Keiko Fujimori accused her opponent’s party of fraud, calling for 200,000 votes to be declared null and a further 300,000 to be scrutinised.

If her request is upheld, the election would be likely to swing in her favour, snatching victory from the hands of her leftwing rival Pedro Castillo. That, in turn, would almost certainly lead to widespread street protests from his supporters.

In a press conference on Wednesday night, Fujimori and her lawyers said they had found evidence of forged signatures on more than 500 ballot tallies along with a host of other irregularities, blaming them on Castillo’s party, Free Peru.

“There are still 500,000 votes at play here, half a million votes nationally, and we think it’s fundamental that they should be analysed before the final count,” Fujimori said. “There’s clear evidence of systematic intent on the part of Free Peru to subvert the popular will.”

Miguel Torres, Fujimori’s lawyer, said her Popular Force party “is not going to throw in the towel” and “will fight until the final vote”. “Many Peruvians feel that their votes are being stolen, and we can’t permit that,” he said.

The accusations, which will go to Peru’s electoral court, came just as authorities finished the count from last Sunday’s election, which passed peacefully and without major incident. The results show Castillo won with 50.2 per cent to Fujimori’s 49.8, a difference of just 72,000 votes. The only tallies left to count are a few that have already been called into question.

Fujimori’s fraud allegations are likely to increase tensions after what was already a bitter campaign between rivals from opposite ends of the political spectrum. Castillo is a leftwing former primary school teacher from a poor rural community in the northern Andes, while Fujimori, making her third bid for the presidency, is from Lima’s political establishment, the daughter of the country’s former authoritarian leader Alberto Fujimori.

Castillo wants to turn Peru’s economic model on its head, saying it has failed the poor, while Fujimori largely defends it.

Financial markets are waiting anxiously for the final result. The currency, the sol, plus Peruvian stocks and bonds have all fallen in recent weeks in anticipation of a Castillo victory, with some wealthy Peruvians scrambling to move their money out of the country.

When initial results came in on Sunday night, supporters of both candidates took to the streets to celebrate what they claimed as victory while accusing their opponents of trying to steal the vote. There were some minor skirmishes in the capital, Lima. Since then, however, both Castillo and Fujimori have appealed for calm and urged their respective camps to wait patiently for the final outcome.

Within minutes of Fujimori’s press conference on Wednesday, Castillo issued another placatory message to his supporters.

“Let’s not fall for provocations from those who want to see this country in chaos,” he wrote on Twitter.