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 Cepeda, a long-time senator ​and activist, won just under 41%, the data showed, a difference of some 668,000 votes.

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, ⁠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.

Both Cepeda, a 63-year-old lawmaker, and his ally, sitting President Gustavo Petro, said they would wait for results to be formally reviewed by judges.

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,“ he said.

“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.”

As of 8.15pm local time, about half of the results had been certified, according to official tallies.

De La Espriella rejected ​Cepeda’s hesitancy to accept the results.

“We will defend the homeland with reason or with force,“ De La Espriella told supporters in coastal Barranquilla, speaking from a large boat that pulled ‌alongside an ⁠esplanade on the Magdalena River, referring to Cepeda as Petro’s puppet.

Left faces tough second round

Surveys have suggested Cepeda will face a much tougher contest in the second round, once right-leaning voters no longer have multiple candidates to choose from.

Multiple centrist candidates won small percentages of the vote.

Low turnout at Sunday’s vote may give the candidates room to manoeuvre, however, if they can convince more supporters to vote in the runoff on June 21.

About 58% of the 41 million eligible voters cast ballots ​on Sunday, figures from the registry ​office showed.

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 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 ​De La Espriella’s history ⁠as a lawyer in his speech and called his rival a representative of “mafia fascism.”

De La Espriella, who has legally represented controversial figures including former Venezuelan minister Alex Saab, has warned that Cepeda would ensure the continuation of 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 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 De La Espriella, as did Uribe.