By Mitra Taj

LIMA (Reuters) - Former investment banker Pedro Pablo Kuczynski inched closer to winning Peru's presidency three days after Sunday's election, holding on to his slim but steady lead over Keiko Fujimori with just 1 percent of the votes left to tally.

Fujimori, daughter of Peru's imprisoned ex-president Alberto Fujimori, was trailing Kuczynski by 0.23 of a percentage point Wednesday, a difference of some 40,000 votes.

While those numbers have fluctuated somewhat in the past two days, Fujimori has not tied or surpassed Kuczynski since counting began.

Peru's electoral office, ONPE, under pressure to wrap up its tally, said it would announce results with 100 percent of all countable votes early on Thursday. However, ONPE will not include unclear ballots that electoral panels must still settle.

Statisticians said that even if most remaining unclear ballots came out in Fujimori's favor, they would probably not be enough.

"The volume is no longer so big that it would allow her to, first of all, close the gap, and on top of that, win enough to claim victory," said Manuel Saavedra, director of the only polling firm of three that had put Fujimori ahead in its exit poll.

A group of Fujimori's supporters gathered outside ONPE's headquarters late on Wednesday, chanting "ONPE, listen, we're watching you!" and waving flags in her party's trademark orange.

Fujimori has largely remained out of the public eye since late on Sunday, when she urged her supporters to wait for complete results. Her critics on Twitter and Facebook have pressured her to concede.

But Fujimori's allies said it was too soon.

"I still hold onto that dream" of Fujimori winning, congresswoman Luisa Maria Cuculiza said in a televised interview.

Kuczynski's team said he would likely win and that it was preparing paperwork needed for a transfer of power from President Ollanta Humala.

"But it's all just preliminary, because we're only going to make moves when all ballots are processed," Kuczynski's running mate, Martin Vizcarra, said in broadcast comments.

Fujimori had been the favorite to win the election just a week ago. But Kuczynski, a 77-year-old former World Bank economist and ex prime minister, caught up with her after Fujimori was stung by scandals involving her close advisers and after Kuczynski performed better in a final debate.

The election for many was a referendum on the legacy of Fujimori's father, who was convicted of corruption and human rights abuse but is credited with defeating leftist guerillas.

(Additional reporting by Teresa Cespedes; Editing by David Gregorio and Leslie Adler)