Aides to third-placed Lourdes Flores, who is trailing Garcia by some 100,000 votes, expressed confidence that she would advance to the second round and called on election authorities to include around 1.5 million marred ballots in the count.
Election authorities told Reuters on Wednesday that it would take around 10 days to review and count the marred ballots and a final result would probably not be announced until the end of the month.
Election officials and representatives from Peru's political parties who worked as observers during the vote said the ballots were either illegible or not filled in correctly.
Xavier Barron, Flores' legal representative, said: "We plan to be in the second round. We're not asking for a re-count, we want these ballots included because we believe they will favour us."
With almost 88% of the votes counted, Humala was running first with 30.97% and assured of a place in a June runoff, as no candidate won a majority.
"We plan to be in the second round. We're not asking for a recount, we want these ballots included because we believe they will favor us"
Xavier Barron, Flores' legal representative
Garcia, whose 1985-1990 rule ended in hyper-inflation and surging violence by Shining Path rebels, was second with 24.44%. Flores, a lawyer and former congresswoman favoured by international investors, had 23.37%.
Election officials continued to tally votes for a fourth day, collecting ballots from remote Andean and jungle areas and Peruvians living abroad.
Only around 5% of expatriate votes, which are likely to favour Flores, had come in, officials said.
Flores says she is particularly determined to beat Garcia to the second round because she narrowly lost to the former leader in the race for a runoff in Peru's 2001 election.
Pre-election polls showed Humala would face a tight runoff against Garcia. If Humala were to face Flores, the polls suggested she would win.