* Election tightest for two decades, June 15 run-off seen
* Test of Santos' strategy of negotiating with rebels (Updates with quotes from voters)
By Brian Ellsworth
BOGOTA, May 25 (Reuters) - Colombians voted for a president on Sunday in the tightest election in two decades that may determine whether the country continues peace talks with leftist guerrillas or steps up its battlefield offensive to end a 50-year war.
The vote has largely become a plebiscite on President Juan Manuel Santos' strategy of negotiating disarmament of Marxist FARC rebels to end bloodshed that has killed some 200,000.
Right-winger Oscar Ivan Zuluaga dismissed the talks as pandering to terrorists and suggested he would scrap them in favor of U.S.-backed military campaigns similar to those led by his mentor, former President Alvaro Uribe.
Santos and Zuluaga are polling neck-and-neck following a race marred by accusations of electronic espionage and drug-linked campaign financing. Neither is seen winning enough votes to avoid a June 15 run-off.
"I voted for Santos and the peace process," said state worker Freddy Ruge after voting in Bogota. "He is the only president who can see the peace process through."
Uribe fell out with Santos, 62, when the president launched peace talks with the Revolutionary Armed Forces of Colombia (FARC) instead of sticking to the eight-year strategy of forcing the group's surrender on the battlefield.
Santos appeals to Colombians who hope the guerrillas will lay down arms after seeing top leaders killed and their numbers halved to about 8,000 fighters.
PAST PEACE FAILURES
The talks in Cuba have yielded agreements on three items of a five-point agenda, including one deal just signed in which the FARC agreed to step away from the drug trade.
But Zuluaga has galvanized conservative Colombians who believe the talks will fail like three similar attempts since the 1980s, including a 1999 peace deal that let the FARC bolster its ranks and boost involvement in drugs.
Three other candidates also on the ballot have mostly polled in single digits and are not seen reaching the second round.
While Colombians are desperate to see an end to the killing, many are outraged that guerrilla leaders accused of crimes against humanity could be pardoned or hold political office.
"Security is important to us, we are 100 percent with Zuluaga," said Jose Gomez, 39, as he left a Bogota polling station with his wife and daughter.
Zuluaga has traveled the country with Uribe, from coca farms in the south to cattle ranches in the northwest, reminding voters of how Uribe's confrontation of insurgents gave Colombia its biggest security gains since war broke out in 1964.
Polls show him surging over the last month to catch or even overtake Santos.
Those surveys may not reflect last-minute scandals including accusations that Zuluaga's campaign sought to sabotage the peace talks by hacking the negotiators' emails and allegations that Santos took drug money during his 2010 campaign.
Both deny wrongdoing.
The two candidates broadly back the same economic platform of attracting foreign investment, boosting output of oil and minerals and maintaining cautious fiscal policy that is applauded by Wall Street. (Additional reporting by Julia Symmes Cobb and Peter Murphy; Editing by Helen Murphy, Lisa Shumaker and Mark Potter)