Conservative Ivan Duque has won Colombia's presidential election after a campaign that turned into a referendum on a landmark 2016 peace deal with FARC rebels that he pledged to overhaul.
Duque, 41, polled 54 per cent to his leftist rival Gustavo Petro's 42 per cent with almost all the votes counted, electoral authority figures showed.
Petro, a leftist former mayor and ex-guerrilla, supports the deal.
Tensions over the deal became apparent in the immediate aftermath of Duque's victory after the president-elect lost no time in pledging "corrections" to the peace deal.
"That peace we long for - that demands corrections - will have corrections, so that the victims are the centre of the process, to guarantee truth, justice and reparation," Duque told supporters in his victory speech at his campaign headquarters.
"The time has come to build real change," Duque said, promising a future for Colombians "of lawfulness, freedom of enterprise and equity," after decades of conflict.
Rebels of the Revolutionary Armed Forces of Colombia, FARC, arrive on top of a truck to El Diamante, in southern Colombia, Friday, Sept. 16, 2016. Source: AAP
His vanquished opponent Petro promised to resist any fundamental changes to the deal.
"Our role is not to be impotent and watch it being destroyed," he said.
The Revolutionary Armed Forces of Colombia (FARC), which disarmed and transformed into a political party after the peace deal but did not contest the election, immediately called on Duque to show "good sense" in dealing with the agreement.
"What the country demands is an integral peace which will lead us to the hoped-for reconciliation," the FARC said in a statement after Duque's presidential win.
The former rebels also called for an early meeting with Duque.
"One of the big questions here is what's going to happen with the peace process," analyst Yann Basset of the University of Rosario told AFP.
"He has said he will not end the agreement, but that he will make modifications, and it's not very clear what these changes will be."
Latin America's longest-running conflict left more than 260,000 people dead, nearly 83,000 missing and some 7.4 million forced from their homes.
The presidential candidate of the 'Movimiento progresistas', Gustavo Petro, in a debate in Bogot, Colombia (Photo by Daniel Garzon Herazo/NurPhoto) Source: AAP
Momentous elections
"These are momentous elections," President Juan Manuel Santos, who will step down in August, said as he cast his ballot early in the day.
"Let us continue to build a country at peace, a country of democracy, a country which we all hold dear and to which we all contribute."
His efforts to end the war with the FARC brought him the Nobel Peace Prize, though he is leaving office with record unpopularity in a country of 49 million people.
The world's leading producer of cocaine, the Latin American country continues to battle armed groups vying for control of lucrative narco-trafficking routes in areas FARC once dominated.
Duque's victory means he will be Colombia's youngest president since 1872.
He comfortably won the first round last month, having campaigned on a pledge to rewrite the agreement signed by Santos.
As he voted surrounded by his children, Duque said he wanted to make sure that those who commit crimes "pay for them."
The former economist and first-term senator says he wants to keep ex-FARC rebels from serving in Congress. The agreement allowed the group to transform itself into a political party.
Duque is buoyed by the backing of his popular mentor, former president and now senator Alvaro Uribe, whose two-term presidency from 2002-2010 was marked by an all-out war on the FARC.
Petro, 58, was the first leftist to reach a presidential runoff in Colombia and believed his presence showed the South American country had shed its suspicions of the left, tainted by 50 years of conflict.
"The need to change things is fundamental," he said after voting.
"We are going to build a humane Colombia that is at peace, that is reconciled with itself."
