Londono's illness came just days after United Nations observers certified that his FARC rebels have completely disarmed, under terms of last year's peace deal with the Colombian government.
Rodrigo Londono, better known by his nom de guerre Timochenko, of the Revolutionary Armed Forces of Colombia (FARC), checked himself into a hospital emergency room in the city of Villavicencio shortly after 8am on Sunday with slurred speech and numbness in his arm, doctors said.
Timochenko, who doctors say is 59 years old, suffered an ischemic stroke, a cerebrovascular accident caused by a blockage that deprives part of the brain from receiving blood and oxygen.
If there are no complications Londono could be released in the next 24 to 48 hours.
Colombia's Marxist FARC rebel leader Rodrigo Londono, known as Timochenko, gestures during a news conference in Bogota, Colombia, November 25, 2016.
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Londono, who is in his 50s, has suffered a number of health scares lately, partly the result of a lifetime in jungle trenches.
"Everything is going well", Londono wrote on Twitter. "I also want to thank the medical team for their care".
Last week the leader oversaw the disarmament of his 7,000 or so rebels at a ceremony with President Juan Manuel Santos.
The deal ended a 50-year uprising against various Colombian governments that killed more than 220,000 people.
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