* Polls show election race too close to call
* Election has become referendum on FARC peace talks
* Both candidates are market friendly
President Juan Manuel Santos, who has cast the election as a choice between peace and war, faces right-wing challenger Oscar Ivan Zuluaga, a skeptic of talks, who nudged ahead of the incumbent in last month's first-round ballot.
The candidates are locked in an unpredictable race, that polls show is too close to call and may have the narrowest margin for 20 years. (For more on the vote, see )
Santos, 62, began talks with the Revolutionary Armed Forces of Colombia (FARC) to end a conflict that has killed over 200,000 people and displaced millions. He has made the pursuit of peace the centerpiece of his campaign.
He announced this week that preliminary talks had begun with the nation's second biggest insurgent group, the National Liberation Army.
"Only Santos can bring peace," said Jonathan Pardo, 30, an engineer in Bogota, who plans to cast his vote for the president's re-election. "Zuluaga will bury the process."
Zuluaga, 55, who accuses Santos of negotiating with terrorists, will impose new conditions on the drug-funded FARC like prison terms for serious crimes and a ban on political participation, should he win.
His vow to suspend peace talks after taking office helped win over hard-line opponents of the negotiations, but he softened that stance after his first-round victory in an apparent effort to attract moderate voters.
The election is unlikely to have much market impact because Santos and Zuluaga, both former finance ministers, hold similar pro-business views. Each wants to encourage foreign investment and improve mining infrastructure.
The rebels have said they will not go to jail and refuse to down their weapons until peace is signed. Santos' backers fear the talks, which have reached partial agreements on three of five points, will collapse if Zuluaga wins.
Other Colombians, weary of previous failed attempts to reach peace, say a return to the hard-line policies espoused by Zuluaga and his mentor, ex-president Alvaro Uribe, is the only way to defeat the FARC and get justice for victims.
"I'm voting for Zuluaga, so the guerrillas won't control things," said David Ramirez, 26, who sells snacks and cigarettes from a cart in northern Bogota. "Santos means more insecurity."
Zuluaga won 29.3 percent of the first round vote, while Santos got 25.7 percent. Three other candidates were eliminated following the first round.
Uribe, who remains popular with voters and is considered a political king-maker, once backed Santos, who served as his defense minister, but the two fell out over the negotiations.
The former president has said the talks are a betrayal of Colombians and that a deal could allow rebel leaders, many of whom are wanted for war crimes or drug trafficking, to escape prosecution and be offered seats in congress.