Maduro, the socialist leader and successor to Hugo Chavez, cast Antonio Ledezma's detention as part of efforts to stop a US-backed coup. Opponents scoffed at that as a smokescreen to distract from Venezuela's economic crisis and staged small protests against what they slammed an authoritarian move.
Intelligence agents had seized the 59-year-old mayor at his office on Thursday.
On Friday night, the state prosecutor's office said he had been indicted for conspiracy and would be jailed in the Ramo Verde military prison, where hardline opposition leader Leopoldo Lopez has been locked up for a year.