Colombia's president-elect Ivan Duque on Sunday pledged not to tear the country's peace process "to pieces," but said he would introduce "corrections" to it, raising concerns about its future.
The 41-year-old right-wing senator took 54 per cent of the vote in a presidential run-off, while his leftist challenger Gustavo Petro, 58, received 41.8 per cent.
"Peace in Colombia is what we all long for," Duque told supporters in reference to the 2016 peace deal with former guerrilla movement FARC that was negotiated by outgoing President Juan Manuel Santos.
But he added that peace "will have corrections" to do justice to the victims of the country's 52-year armed conflict.
Petro tweeted he had received so many votes it could not be regarded as a defeat, but that "for now we shall not form a government."
The left was defeated through "lies" such as saying churches would be closed or that Colombia would become another Venezuela, Petro told supporters in a speech broadcast on television, adding: "We shall be the opposition... because we reject the imposition of obstacles on peace in Colombia."
Opinion polls had predicted the victory of Duque, a formerly little-known politician, who was picked by powerful former president Alvaro Uribe as the candidate of his Democratic Centre party.
Petro nevertheless made history as the leftist who came closest to the presidency in the traditionally conservative South American country.
Duque's victory will ensure Marta Lucia Ramirez, 63, becomes Colombia's first female vice president.
Three moderate candidates fell out in the first round on May 27, leaving Duque and Petro facing each other in a bitter run-off.
About 36 million people were eligible to vote in the country's first elections in half a century that were not overshadowed by the armed conflict with FARC.
Voter turnout appeared low, with some people saying they felt reluctant to choose between two candidates representing opposite programmes in a polarized political landscape. More than 4 per cent of voters submitted spoiled ballots.
Duque's victory raised concern that many of the 7,000 demobilized FARC fighters might join smaller armed groups, reigniting a conflict that left at least 220,000 people dead and displaced 7 million.
One of the leftist guerrilla groups involved in the conflict, the 1,500-strong National Liberation Army (ELN), still remains active, along with about 1,200 FARC dissidents and right-wing paramilitary groups.
The post-conflict justice system created for FARC and the army is not adequately punishing former guerrilla commanders for killings and kidnappings of civilians and for cocaine trafficking, Duque has argued.
He may modify the functioning of the transitional justice system and deprive FARC - now a political party - of the minimum of 10 seats in Congress it was guaranteed in the peace deal, constitutional law expert Juan Manuel Charry said.
Duque is also expected to impose stricter conditions on the ongoing peace talks with ELN.
The president-elect has little political experience other than his five-year stint as senator.
Many of his critics jokingly call him "Uribito" ("little Uribe") in a reference to Uribe who they say will pull the strings after his protege becomes president.
Uribe, who was president from 2002 to 2010, campaigned vehemently against the peace deal ahead of a referendum that was held on it in October 2016.
A very slight majority of Colombians voted against the agreement, but Santos made some modifications to it and pushed it through Congress.
Duque's eventual attempts to change the peace deal are now expected to be restricted by Congress and by the Constitutional Court.
The right-wing candidate also focused much of his electoral campaign on warnings that a leftist victory would turn Colombia into another Venezuela, which is suffering a severe economic crisis under socialist President Nicolas Maduro.
Petro, a former member of the now extinct guerrilla group M-19 and erstwhile Bogota mayor, had pledged policies of state-sponsored universal health care and education to bridge the wealth gap in one of the region's most unequal countries.
Nearly 160,000 police and soldiers were deployed to guarantee security during the elections, described as Colombia's most tranquil ever by the electoral authority.
Santos, who won the 2016 Nobel Peace Prize for his efforts to seal peace with FARC, was barred by the constitution from seeking a third term in Office.
