"The diagnosis for the patient was a heart attack. His condition is stable and he is fully conscious," said doctor Andrey Rojas of the medical center in the town of Fusagasuga near the capital Bogota.

Londono is a candidate to be president of Colombia in an election due on May 27, though the 59-year-old is polling with just one percent voter support.

Since a historic peace deal struck with the government of outgoing President Juan Manuel Santos in late 2016, the Revolutionary Armed Forces of Colombia (FARC) has given up its half-century armed struggle and transformed itself into a political party.

The group, now called the Common Alternative Revolutionary Force, is aiming to secure legislative seats given to it under the peace accord in elections for Colombia's bicameral congress in May.

But last month, Londono and his party suspended their campaigns, saying there were insufficient security safeguards for FARC candidates.

Security personnel have had to protect Londono from people trying to assault him at campaign events or upon exiting media interviews -- a sign of the passions left over from the long and brutal conflict.