Opinion polls show 41-year-old senator Ivan Duque as the favorite, buoyed by support from the Democratic Center party of hardline ex-president Alvaro Uribe.
The latest opinion polls show Duque with 41 percent of voter preferences, some 12 points ahead of his nearest challenger, the leftist former Bogota mayor Gustavo Petro.
If the opinion polls are confirmed, neither candidate can win outright on Sunday and will face each other in a run-off on June 17.
That alone would be a staggering performance for Petro, a former member of the now disbanded M-19 rebel group, making him the first leftist politician to reach a second round in conservative dominated Colombia in recent years.
The vote is taking place against a background of tentative change in the Latin American country where the peace deal, in effect for little more than a year, remains fragile.
The FARC has transformed into a political party which has thus far failed to win much popular support, but Colombia -- gripped by corruption and glaring inequality -- is still struggling to emerge from the longest armed conflict in the Americas.
The world's leading producer of cocaine, the country remains contorted by an ongoing struggle against a slew of armed groups vying for control of lucrative narco-trafficking routes in areas once dominated by FARC guerrillas.
Step towards peace
President Juan Manuel Santos, who forged the 2016 peace deal, said the election is "very, very important for this new Colombia that we are building, a Colombia in peace."
Santos announced the deployment of an extra 155,000 troops to ensure security for the election, for which the country's last active rebel group, the ELN, has announced a ceasefire.
A former economist with the Washington-based Inter-American Development Bank (IDB), Duque is still something of an unknown quantity after only four years in politics as a senator.
"As a politician, he is little known because he is under the wing of Uribe...Nobody knows yet whether he has his own ideas or if he will obey orders," said analyst Fabian Acuna of Javeriana University.
Duque wants to rewrite the peace treaty, which he believes was too lenient on former guerrilla leaders who waged a decades-long campaign of violence against Colombians.
Although the FARC polled only 0.5 percent in legislative polls, it was guaranteed 10 seats in parliament under the peace deal.
"What we Colombians want is that those who have committed crimes against humanity be punished by proportional penalties, which is incompatible with political representation, so that there is no impunity," Duque told AFP.
The young senator has pledged to eradicate "the cancer of corruption" and work to revive a sluggish economy, in campaign speeches which focused on the defense of traditional family values.
No more fear of the left
Petro drew bigger crowds to his rallies the longer his campaign went on, in what some analysts see as public support for the peace deal and the rehabilitation of FARC as a political party.
"Society has overcome the fear of violence and terror, and what we are seeing today is the political expression of that, filling public places and drawing crowds," Petro told AFP in the final days of his campaign.
"His rise can be explained by the great polarization in the country, he being one of the polar opposites," said analyst Andres Macias, of Externado University.
Petro defends the peace deal with the FARC and continuing dialogue with the ELN. Duque has said he intends to suspend negotiations with the ELN.
But Petro has had to push back against attempts to taint him by association with Venezuelan President Nicolas Maduro.
His critics, including Duque, have been referring to him as a "Castro-Chavista", a reference to late Cuban president Fidel Castro and late Venezuelan president Hugo Chavez, and accuse him of being likely to lead Colombia down the road to populist ruin as Maduro has in neighboring Venezuela.
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