The Trump-admiring far-right millionaire lawyer and self-styled “outsider” Abelardo de la Espriella has won Colombia’s presidential runoff, defeating the leftwing senator Iván Cepeda.

With 99.65% of ballots counted in the preliminary vote tally, de la Espriella had secured 12.91m votes, or 49.65%, just 248,310 more than Cepeda, who received 12.67m votes, or 48.7%. A further 1.6% of ballots were cast blank.

The margin was narrower than in the first round three weeks ago, when de la Espriella had beaten Cepeda by 673,000 votes.

De la Espriella’s victory marks a sharp swing back to the right after four years under Colombia’s first and only leftwing president, Gustavo Petro, who was barred by the constitution from seeking re-election and therefore backed Cepeda as his successor.

The result is also being seen as further evidence of a wave of far-right candidates sweeping presidential elections across Latin America, after recent victories by Nasry Asfura in Honduras and José Antonio Kast in Chile, while Keiko Fujimori currently leads the vote count in Peru.

Like them, de la Espriella also received the endorsement of the US president, Donald Trump – although only after winning the first round.

When Petro leaves office in about six weeks, only Mexico, Brazil – which will hold elections in October – Uruguay and Guatemala will remain under leftwing governments in the region.

In a post on social media, Petro alleged irregularities in the preliminary vote count released by the National Civil Registry, the independent public body responsible for organising Colombia’s elections. Without providing evidence, the president claimed that the registry was allegedly “uploading forms … without the signatures of election jurors” and said that “those polling stations must be immediately challenged”.

Petro also wrote that he would only recognise the outcome of the official scrutiny process, which is expected to take about two more days. “No president can be declared yet. It is the scrutiny process that determines who the president is,” he said.

In the first round, the president also alleged fraud in the preliminary count without presenting evidence, drawing widespread criticism from election experts. Historically, the difference between the preliminary count and the official scrutiny in Colombian elections has been less than 1%.

In a campaign dominated by the violence that has once again engulfed the country, de la Espriella prevailed on a promise to adopt an iron fist approach against criminal groups.

Although security indicators remain far below the extraordinarily high levels recorded in the decades before the landmark 2016 peace agreement between the government and most of the Revolutionary Armed Forces of Colombia, the past year has been the most violent since then.

The president-elect, who will take office on 7 August, has pledged to build 10 maximum-security “mega-prisons” and kill criminals “like rats and cockroaches”. He has also promised to “disembowel” the left – a remark he later said was merely a figure of speech.

Calling himself El Tigre (the Tiger) and having never held public office, de la Espriella has vowed to make a complete break with Petro’s “total peace” plan of negotiating the dismantling of all criminal groups.

After four years of fits and starts – during which analysts say some armed factions took advantage of temporary ceasefires to continue expanding – the government managed to disarm the first criminal group only on Thursday, one with just 99 members, while experts estimate that more than 27,000 people belong to Colombia’s many criminal organisations.

The new president, by contrast, has promised a return to full-scale military confrontation that has done little to curb violence in the past, and said he will seek US support for airstrikes against coca plantations. Colombia is the world’s largest producer of cocaine, and drug trafficking is the main driver of the country’s violence.

Born in the capital, Bogotá, but raised on Colombia’s Caribbean coast, de la Espriella rose to prominence as a criminal lawyer representing the leaders of a group at the heart of the decades-long armed conflict: the paramilitaries, private armies created by rightwing landowners to fight leftwing guerrilla groups.

Later branching out into liquor, real estate and menswear businesses, and fond of showcasing a lavish lifestyle on social media, he announced his presidential bid in July last year, a month after the rightwing senator and presidential hopeful Miguel Uribe Turbay was shot during a campaign event – he died two months after the attack.

Though long associated with Colombia’s rightwing political establishment through his legal career, de la Espriella presented himself as an “anti-establishment” candidate, following the example of many other far-right leaders who have risen to power across the region in recent years.

His vice-president will be the economist José Manuel Restrepo, who served as finance minister under Petro’s conservative predecessor, Iván Duque. The president-elect said that Restrepo would be responsible for implementing the plan to shrink the state by 40%.

They will take office with a minority in congress and what many analysts see as a deeply divided country after the most polarised election in years, in which the two candidates failed to agree on holding a single debate and instead traded a barrage of insults.