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By Peter Murphy
BOGOTA, June 5 (Reuters) - Colombia's President Juan Manuel Santos and rival Oscar Ivan Zuluaga were neck-and-neck in a Gallup poll of voters published on Thursday, before a June 15 run-off election centered on how to end 50 years of civil war.
Right-wing economist Zuluaga had a narrow lead with 48.5 percent, versus 47.7 percent for the president, but the pair are effectively tied given the poll's 3 percent margin of error.
The figures will intensify a bitter election battle shaping into Colombia's closest for two decades.
Incumbent Santos is fighting for a second four-year mandate to complete peace talks he initiated with the leftist FARC or Revolutionary Armed Forces of Colombia in 2012. The talks, he says, offer a rare chance to end a war that has killed around 220,000.
Zuluaga wants to impose tougher conditions for those talks in Havana, which would heighten the risk of their collapse.
His demands include a FARC ceasefire, strict adherence to a timetable, jail terms for leaders, and an end to the planting of land mines and forced recruitment of children.
While Santos had long been favored to win the election, Zuluaga has won increasing support from voters who question the FARC's willingness to disarm and walk away from the lucrative illegal drugs trade even if a peace deal is signed.
Zuluaga, an economist, won the most votes in the first round of the election on May 25, coming in 3.6 percentage points ahead of Santos.
The Gallup survey of 1,200 people carried out from May 31 to June 3 and published in local media, came before Clara Lopez, a leftist candidate knocked out of the race in the first round, endorsed Santos this week.
The election campaign in Latin America's fourth-largest economy has been a bitter one.
The campaign saw video footage apparently linking Zuluaga to spying to obtain secret government information useful to his campaign, and accusations against Santos that drug money funded his 2010 presidential campaign.
Both candidates deny any wrongdoing. (Additional reporting by Luis Jaime Acosta; Editing by Andrew Cawthorne and Lisa Shumaker)