Ivan Cepeda, an ally of Colombian President Gustavo Petro, refused to immediately accept the result of the first round of voting after falling behind a tough-on-crime outsider Aberaldo de la Espriella in Sunday's election.
Cepeda and de la Espriella are slated to head to a run-off election June. But Cepeda and Petro on Sunday night sowed doubt in the result and claimed without evidence that hundreds of thousands of votes were manipulated and that foreign actors manipulated the results of the election.
Cepeda said he was waiting for electoral authorities to scrutinise the results before accepting the election.
"We will not comment on tonight's results until the vote-counting committees have fully clarified this matter," Cepeda said.
Cepeda won 41 per cent of the vote, while de la Espriella won 44 per cent of the votes, according to electoral authorities.
Cepeda was consistently leading polls in the run up to the Sunday vote, but in the weeks leading up to the election de la Espriella rapidly gained support with a promise that he would crack down on armed groups.
Despite that, he appeared to acknowledge that the vote was going to a runoff election, saying "we are going to win in the second round."
Lawyer Abelardo de la Espriella and peace-builder Ivan Cepeda were leading the vote counts in the first round of Colombia's presidential elections on Sunday, and are to face off in a presidential runoff in the South American nation later in June.
De la Espriella, a newcomer known as "El Tigre, or "The Tiger" who has sought to portray himself as a tough-on-crime supporter of President Donald Trump, is leading the race with more than 43 per cent of the votes after nearly 98% of the ballots were counted, Colombia's electoral authorities said on Sunday night.
He fell short of the 50 per cent of votes needed to win in the first round of voting, something that may spell trouble for his path to the presidency.
He was tailed by Cepeda, progressive senator and an ally of outgoing President Gustavo Petro who has promised to carry on a fraught push for "total peace." Cepeda scooped up less than 41% of the vote.
Paloma Valencia, a candidate for Colombia's establishment party who pitched herself as a centrist, fell short of moving on to the next round with less than 7 per cent of the vote.
While Cepeda coasted comfortably ahead in polls throughout the campaign, the senator and de la Espriella were neck-and-neck on Sunday night, likely to spell trouble in the next round of election in June, where de La Espriella is likely to scoop up many of the voters that threw their support behind Valencia.
The results in the second round of voting later this month are slated to set the South American nation on two sharply diverging paths, acting as a sort of compass for political shifts in Latin America at a time that the region has increasingly swung to candidates that have pushed for a more militaristic crackdown on criminal groups and aligned themselves with President Donald Trump.
On one path, Cepeda has promised to continue Petro's progressive agenda and fraught push of trying to negotiate peace pacts with armed groups, following a plan that's likely to sharply contrast with Trump's vision for Latin America.
On another, Espriella has promised to fiercely crackdown on criminal groups, following in a similar vein as El Salvador's President Nayib Bukele in his war on gangs, which has fueled accusations of human rights abuses.
In Colombia, a historic rightwing stronghold, Valencia's electoral loss dealt another blow to a once powerful political current known as Uribismo, indicating that conservative voters are turning away from more traditional political parties in favor of Bukele and Espriella's punitive populism.