Former Peruvian army officer Ollanta Humala claimed victory late Sunday in Peru's tightly-contested presidential runoff election, ahead of a complete count of official results.

Partial results gave Humala a razor-thin lead over Keiko Fujimori, the daughter of imprisoned former president Alberto Fujimori.

A count of selected ballots found Humala leading with 51.5 percent of the vote.

Participating in elections is mandatory for Peru's nearly 20 million eligible voters.

In the first round of balloting in April, Humala won 32 percent of the vote, falling short of the majority needed for an outright win. Fujimori, a conservative candidate, took 24 percent.

Some voters have expressed concerned that as president, Fujimori would try to free her father, who is serving a 25-year prison sentence for his role in death squad killings in the 1990s. Fujimori has apologized for mistakes and crimes committed while her father was president from 1990 to 2000.

Humala led an uprising against Alberto Fujimori in 2000, but lost a runoff election to current President Alan Garcia in 2006. Humala was outspoken during that campaign about his admiration for Venezuelan President Hugo Chavez, but he has since distanced himself from the leftist leader.

Much of the latest presidential campaign focused on continuing Peru's rapid economic growth of recent years, while ensuring that the poor also see some of that increased prosperity.