Popeye, 56, who was released from prison in 2014 after 23 years, was already under investigation for allegedly making threats on his Twitter account against Gustavo Petro, leftist candidate in Sunday's election.
But he was arrested for "extortion," a source in the office of the special prosecutor in charge of fighting organized crime told AFP.
Popeye was one of Escobar's chief "sicarios" -- or hitmen -- during the bloodiest period of the Medellin Cartel's reign in the 1980s.
But he parted ways with the Cartel in 1992. Escobar was killed in a rooftop shootout with police in Medellin the following year.
Since leaving prison, Popeye has been very active on social media.
A leftist senator, Ivan Cepeda, informed police about the threatening messages sent to Petro and his supporters.
"We received a formal complaint in this case" and launched an investigation, said chief prosecutor Nestor Humberto Martinez.
Popeye has said that his Twitter account had been hacked.
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