LIMA (Reuters) - Keiko Fujimori conceded defeat to centrist Pedro Pablo Kuczynski in Peru's tightest presidential election in decades, but said on Friday that her populist party's solid majority in Congress would wage a "watchful" opposition during his government.

Fujimori, the daughter of the country's imprisoned ex-president, said that former investment banker Kuczynski had only won the run-off race with the support of "promoters of hate."

"In the second-round our opponent was joined by the political power of the outgoing government, economic powers and media power," Fujimori said in her first press conference five days after Sunday's election.

Her party's 73 incoming lawmakers to the single-chamber, 130-seat Congress applauded her under a banner that read "Thank you, Peru!"

It was the second straight narrow loss in a run-off election for Fujimori and a further blow to the populist politics that have fallen out of favor in Latin America.

Fujimori, 41, had been the favorite to win the election just two weeks ago, following years of pitching her iron-fist approach to crime in Lima's gritty districts and promising public works in far-flung provinces.

But septuagenarian Kuczynski scraped together a narrow win after stepping up his attacks on her as her close advisers were mired in scandal in the final days of campaigning, despite his stiff technocratic style and worries about his age.

While Fujimori said she accepted official results that showed Kuczynski ahead of her by only tens of thousands of votes, she called them "confusing."

Members of her party had cast doubts on the vote count on Thursday, accusing electoral authorities of counting votes from polling stations where they said their representatives had detected irregularities.

(Reporting By Mitra Taj and Teresa Cespedes; Editing by Chris Reese and Alistair Bell)