A far-right lawyer and Donald Trump admirer will go head-to-head against leftwing senator Iván Cepeda in the race to be Colombia’s next president after he won a surprise victory in the first round of voting.

With 100% of ballots counted, the outsider Abelardo de la Espriella secured 43.7% of the vote – just over 10.3m votes – compared with 40.9% (about 9.6m votes) for Cepeda, a philosopher and human rights activist who has served as a senator since 2014 and is backed by the current leftwing president, Gustavo Petro.

De la Espriella, who calls himself el Tigre (the Tiger), celebrated the result: “Compatriots, defenders of the homeland, more than 10 million Colombians placed their trust in el Tigre and joined the pack ... In 21 days, we are going to change the history of Colombia forever,” he said in a video alongside his wife and children, all wearing shirts of the Colombian national football team.

After a wave of victories by far-right candidates in recent years in Argentina, Chile, Ecuador, Bolivia and Honduras, Colombia remains one of the few countries in Latin America still governed by the left, alongside Mexico and Brazil, which will hold its own presidential election in October.

Although polls in recent weeks had already detected De la Espriella’s rapid rise, most still showed him trailing Cepeda, who for months seemed to hold a solid lead. The two will face each other in a runoff on 21 June.

De la Espriella is an outspoken admirer of several rightwing leaders in the region, including the US president, Donald Trump, El Salvador’s Nayib Bukele and Argentina’s Javier Milei.

He appears to have consolidated much of the vote that had previously been going to the rightwing senator Paloma Valencia, who at one point polled above 20% and was running in second place but finished Sunday with just 6.9%.

Petro posted on X that “as president, I do not accept the preliminary results” released by the National Civil Registry, the independent public body responsible for organising Colombia’s elections.

Without showing any evidence, the president claimed the count included “800,000 additional people” and said he would only “consider and accept” the results of the official scrutiny process, during which the National Electoral Council reviews the physical tally sheets, a procedure that can take days or even weeks.

The lawyer Juan Carlos Galindo Vácha, who previously headed the National Civil Registry on two occasions, accused Petro of spreading “disinformation”.

“Historically, in presidential elections, the difference between the preliminary count, which is unofficial, and the official scrutiny process is less than 1%. That alone undermines any claim by President Petro that there was fraud in the count,” he said in an interview with Radio Caracol.

He added: “The president should show greater respect for the citizens who take part in the electoral process, whether as polling officials or electoral observers. He should not make these wild claims that even he does not understand.”

Cepeda delivered his speech shortly after Petro’s post and echoed the president’s allegations, likewise without presenting evidence. The senator said there was “information regarding a certain number of polling stations” in which “atypical voting patterns” had allegedly occurred. “Only once the electoral commissions have fully clarified this matter will we comment on tonight’s results,” he added.

A criminal lawyer and millionaire businessman who has never held public office, De la Espriella built his campaign around a promise to return to a policy of total confrontation in response to Colombia’s worsening security crisis, now considered the worst since the landmark 2016 peace agreement between the government and most of the Revolutionary Armed Forces of Colombia (Farc).

De la Espriella advocates ending Petro’s “total peace” policy of negotiating the dismantling of criminal groups – of which Cepeda is widely regarded as the architect – and replacing it with a mano dura (iron-fist) strategy inspired above all by El Salvador’s populist strongman Bukele, who has imprisoned at least 2% of his country’s adult population as part of a controversial crackdown on gangs. Even the lawyer’s neatly trimmed beard and habitual use of baseball caps have drawn comparisons with Bukele’s style.

De la Espriella has incorporated his tiger nickname into much of his campaign branding. He has attracted controversy by attacking journalists and, at one point, telling a radio host that he was winning over female voters because of the size of his genitals.

In a speech on Sunday night, Valencia acknowledged the result and endorsed De la Espriella in the runoff.

Despite widespread concern about security, election day itself passed peacefully.

The past few months have been marked by a surge in guerrilla attacks, homicides, kidnappings, forced displacement and massacres, and last year the rightwing senator and presidential hopeful Miguel Uribe Turbay was shot during a campaign event by a Farc dissident group and later died.