Ex-presidents Alvaro Uribe and Andres Pastrana say they are focusing their platform on objections to the peace deal signed with the Marxist FARC rebels.
Ledezma is the best-known detained opponent of leftist President Nicolas Maduro after Leopoldo Lopez, and had spearheaded street protests against the president in 2014.
The revised peace deal was put through Congress after voters shocked the world by rejecting an earlier version in a referendum last month.
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