The protest in Lima began in San Martin Square and proceeded through the historic center of the city, passing by the front of Congress.
Under the banner of "Keiko Out, Fujimori Never Again," the peaceful demonstrators held signs with slogans against the candidate and her late father, former Peruvian president Alberto Fujimori, an AFP journalist observed.
At the front of the march were relatives of victims of human rights violations during the elder Fujimori's presidency between 1990 and 2000.
"Keiko represents injustice and fear. Keiko Fujimori shouldn't become president because, for us, she is a danger. If she wins, it will be like a fraudulent election," Irma Cayo, 30, told AFP.
In addition to the protest in Lima, there were demonstrations in the cities of Arequipa and Huancayo.
The June 7 runoff vote is expected to be as heated as the 2021 ballot, which also involved Fujimori against leftist Pedro Castillo, who won the vote but was removed from office in December 2022.
The first round of voting on April 12 featured a record 35 candidates vying for the presidency, with Fujimori ultimately gaining 17.1 percent of the vote, while runner-up Pedro Sanchez captured 12 percent.