The ceasefire is coming after four years of peace talks in Cuba between the Farc and the government. The final agreement on ending the 52-year-old war will be signed next month.
Farc leader Rodrigo Londono, known as Timoleon Jimenez or Timochenko, gave the order to stop firing.
"Never again will parents be burying their sons and daughters killed in the war,'' Timochenko told journalists. "All rivalries and grudges will remain in the past," he said.