Colombians vote Sunday in a divisive presidential election likely to impact on the government's fragile peace deal with the former rebel movement FARC.
Conservative front-runner Ivan Duque has vowed to rewrite an accord he sees as too lenient on a group that waged a decades-long war of terror on Colombians, before it transformed into a political party.
With 41 percent of voter preferences, Duque takes a 12 point lead into the election over his main challenger Gustavo Petro, a leftist former Bogota mayor who backs the deal.
Petro, a former member of the disbanded M-19 rebel group, has been the surprise package in the campaign, upturning expectations in a country where presidential elections have traditionally been the domain of the right.
The 58-year-old rallied many Colombians with his campaign speeches against inequality and corruption, making him the country's first leftist candidate with a chance of going to the second round of a presidential poll.
Neither candidate is believed to have enough momentum to win outright in Sunday's first round, making a runoff likely on June 17.
The winner will succeed Juan Manuel Santos -- the president who signed the 2016 deal with the FARC and was awarded the Nobel Peace Prize -- when he steps down in August.
"These elections have an enormous significance for a country where hope is reborn, and we must continue to build peace," Santos, 66, said on Twitter as campaigning ended.
Duque, a gray-haired senator and former economist, is backed by the Democratic Center party of former president Alvaro Uribe, which swept the polls in legislative elections in March. If successful, he can count on the support of Congress.
Uribe fell out with once-close ally Santos over his drive for peace with FARC, setting up his own party in 2013.
Many voters see the guiding hand of Uribe behind the inexperienced Duque's campaign. Like Santos, the 65-year-old Uribe is constitutionally precluded from seeking a third term.
"If Duque was the candidate of another party, he would not be leading the polls," said analyst Andres Macias of Externado University, highlighting the "great political capital" of the controversial but popular former head of state.
Colombia, ravaged 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.
The peace deal, in effect for little more than a year, remains fragile. The FARC has transformed into a political party which failed to win much public support in March legislative elections, failing to add to the 10 parliamentary seats it was awarded as part of 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 during campaigning recently.
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.
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.
Analysts say the men are at opposite ends of the political spectrum and are likely to prevail over the four other candidates, who include former vice president German Vargas Lleras.
"Two candidates are clearly in the lead and, except for a surprise or a big error in the polls, Petro and Duque will go to the second round, although there is also the possibility that Petro wins outright," political scientist Nicolas Liendo told AFP.
The country's last active rebel group, the National Liberation Army (ELN), has announced a ceasefire for the polls. Nevertheless, security has been tightened, with 150,000 troops being deployed.
Polls are due to open at 8:00 am (1300 GMT) and close at 4:00 pm.