(Adds quote from FARC statement, details on elections)
BOGOTA, June 7 (Reuters) - Colombia's leftist FARC rebels declared a ceasefire on Saturday from June 9 to 30, a period that will cover a run-off presidential election being contested mainly over how to end five decades of war with the guerrillas.
The three-week ceasefire was announced in an open letter posted on the group's website and written to right-wing presidential candidate Oscar Ivan Zuluaga who has been skeptical about peace talks with the group begun by his rival and incumbent President Juan Manuel Santos.
"We want to tell the country, through you (Zuluaga), our decision to declare another unilateral ceasefire for the occasion of the second round of elections to the Presidency of the Republic," said the letter signed by FARC leader Timoleon Jimenez, known as Timochenko.
Zuluaga is the anointed candidate of popular ex-president Alvaro Uribe, who led an unprecedented U.S.-backed military assault against the FARC that is estimated to have halved their ranks to around 8,000.
President Santos began negotiations with the FARC in the Cuban capital Havana in late 2012 with an agenda now roughly half complete. The FARC are still combating government troops in their rural hideouts, having rejected a unilateral ceasefire unless the government reciprocates.
Nonetheless, the FARC declared a week-long ceasefire around the period covering the first round of elections on May 25 in which Zuluaga won most the most votes of any candidate but fell far short of the 50 percent of votes needed to avoid a run-off.
Three voter polls on Thursday and Friday showed the second round vote between Zuluaga and Santos will be very tight. A Gallup poll said the two were in a dead heat while two others gave a solid lead to one candidate or the other.
The election campaign has largely divided voters between those who back Santos to give him a chance to complete the peace talks and Zuluaga supporters who are more doubtful that the rebels will disarm and walk away from the lucrative drug trade even if a deal is signed. (Reporting by Peter Murphy; Editing by Eric Walsh)