The June 9-30 ceasefire was announced in a scathing open letter to opposition candidate Oscar Ivan Zuluaga, posted on the group's website, in which it accuses him of making inflammatory statements and trying to foment more war.
The FARC are keen to avoid Zuluaga's election, analysts say, because conditions he would impose to continue peace talks could lead to their breakdown and a return to all-out war. The talks were started in Havana in 2012 by 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, who is known as Timochenko.
The long letter, announcing the ceasefire only near the end, appeared designed to convince voters a week before elections that Zuluaga would undo progress towards peace achieved under Santos and reduce hopes of ending the war if voted into power.
The election has divided voters between Santos backers who want to give more time to peace talks and Zuluaga supporters who doubt the rebels will disarm and walk away from the lucrative illegal drugs trade if a deal is signed.
Three voter polls on Thursday and Friday failed to predict a clear winner in next Sunday's election. A Gallup poll put the two in a dead heat, while two others gave a solid lead to one candidate or the other.
Zuluaga demands a FARC ceasefire, jail terms for FARC leaders and an end to the planting of land mines and forced recruitment of children in order for the talks to continue.
The head of the FARC negotiation team in Havana told reporters on Saturday the delegation felt they were getting closer to "the summit, the Mount Everest of rights, which is peace".
Despite opening negotiations with the FARC, the government has continued to pursue the rebels militarily in their jungle hideouts and the FARC rebels have refused to declare a ceasefire unless the government reciprocates.
Nonetheless, the FARC had also declared a week-long ceasefire around the period covering the first round of elections on May 25, in which Zuluaga won more votes than other candidates but fell far short of the 50 percent of votes needed to avoid a run-off.
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