Colombian right-wing outsider Abelardo De La Espriella is set to compete in a runoff election for president against leftist Ivan Cepeda, results from an initial round of voting showed, as Mr Cepeda and his allies said they would wait for counts to be formally confirmed.
The two men were separated by less than two percentage points, data from the country's national registry office showed, in a contest focused on security, the economy and populist policies.
Lawyer De La Espriella received 43.7% of the votes and Mr Cepeda, a long-time senator and activist, won just under 41%, the data showed, a difference of nearly 668,000 votes.
Mr De La Espriella, who has never held elected office, has drawn comparisons with El Salvador's Nayib Bukele over his style and policy proposals.
Portraying himself as an outsider free from political baggage, Mr De La Espriella, 47, has proposed a tough offensive against illegal armed groups, the construction of 10 megaprisons and poverty reduction through better education, healthcare and housing for the poorest.
Mr Cepeda, a 63-year-old lawmaker, has been leading opinion polls, but surveys suggested he will face a much tougher contest in the second round, once right-leaning and centrist voters no longer have multiple candidates to choose from.
Low turnout at yesterday's vote may give the candidates room to manoeuvre, however, if they can convince more supporters to vote in the runoff on 21 June.
About 58% of the 41 million eligible voters cast ballots, figures from the registry office showed.
Left rejects initial results
Both Mr Cepeda and his ally, sitting President Gustavo Petro, said they would wait for results to be formally reviewed by judges.
Mr Cepeda told supporters in Bogota that irregularities may have occurred at an unknown number of polling stations.
"We are verifying, through our security and electoral observation mechanism, exactly how many are involved, according to initial reports, atypical voting has occurred. We therefore make it clear to the public that only once the vote-counting commissions have fully clarified this matter - clearly and rigorously - will we issue any statement on tonight’s results," he said.
Mr Cepeda, the son of a murdered communist leader, has promised to pursue peace with illegal armed groups through negotiations, an approach that has brought little progress under Mr Petro.
He also plans to deepen reforms meant to reduce inequality and poverty, including by raising taxes on high-income earners, granting 1 million hectares to victims of the country's six-decade internal conflict and expanding healthcare coverage.
He decried Mr De La Espriella's history as a lawyer in his speech and called him a representative of "mafia fascism".
Mr De La Espriella, who has legally represented controversial figures including former Venezuelan minister Alex Saab, has warned that Mr Cepeda would ensure the continuation of Mr Petro's economic policies, including a ban on new oil projects, which have drawn criticism from establishment politicians and investors.
The lawyer says he has financed his campaign with his own resources, without receiving donations from parties or large companies. Reuters could not independently verify that claim.
Paloma Valencia, a senator backed by former President Alvaro Uribe, had until recently been the leading right-wing candidate in the race, but she captured fewer than 7% of the votes. She said she will back Mr De La Espriella, as did Mr Uribe.
